Just out of curiosity I am wondering how many words do I have to write in order to get to the next act? Is there a rule among writing stage plays or can you do whatever you want when it ultimately comes down to writing one?
How long do you want the play to be? How many characters are there? There's so many factors here.
I'm a season ticket holder for equity-level theatre for many decades. I see a lot of plays, and have written a couple. I urge you to attend theatre where you are, at whatever level is available, including high school and college productions. (Skip the musicals unless that's your focus.) You'll get a better feel for what works on stage as well as pacing.
Most modern plays have either two acts or occasionally three. The first act is generally longer and ends with what would be a page-turner if it were a book--something important implied or revealed.
Because playwrights follow a relatively rigid format (something like screenwriting, but with a bit more flexibility) and required font and size, they can count on their pages averaging one minute of performance time. A full-length play tends to be a minimum of sixty pages, but more often comes in between eighty and ninety. The first act is around ten to fifteen minutes longer than the second (or the second and third combined).
This is to allow for an intermission during which the audience can use restrooms--and the theatre can make money selling food and beverages. The length of the intermission depends almost entirely on the lines at the ladies' room. They rarely resume performance until everyone's done in there.
The one-act play cannot exceed ninety minutes, BTW. (We attended a one-act last weekend and people had to get up during the performance, which is pretty disruptive.)
So it's not word count but page count, if and only if you're using a standard format, font, margins, etc. In a play that's as short as yours, the placement of the acts isn't going to matter as much as a full-length play, because there will be no need for intermission. You'd just want to follow a standard structure of establishing who's who and what's going on, introducing a problem or event, etc., just as you would with a novel.
You might be hard-pressed to find placement for a thirty-minute play. Some theatres have ten-minute play festivals, but I've never seen one that length performed. After you finish it, consider whether there is a way to expand it?
Having five characters is excellent. For most local and/or small theatres, the cost of production is a major factor. Fewer actors to pay is good. (If your play allows it, look for the same actor playing two roles, differently costumed.)
Minimal set and prop requirements is also a selling point; I once had a play rejected because they couldn't afford to make a mock-up of an old-style camera on a tripod, with a draped cloth, necessary for the play to work. It might have cost $50. That's how shoestring theatre companies' budgets can be.
Maryn, hoping this helped
When I tried a play, I had a lot to learn. I hope to pass it along, making it easier for the next person. Which is why I'm pasting this just for you to copy and use. Please don't post it online anywhere else. (Not that you would have.)
PLAY FORMAT
While you can use a program that does it for you, it's easy enough to do it yourself. There are lots of minor variations; plays are less rigid than screenplays.
Margins: Set the left margin a 1.5 inches from the edge of the paper. Set the right margin at 1.0 inches from the edge of the paper. This gives you a line length of 6.0 inches and allows all the text to be easily read when the left edge is bound.
Font: Courier New or any other Courier, 12 point/size 12, which produces ten characters per inch and, amazingly enough, averages one minute per page, same as a screenplay. (If you use a proportional font like Times New Roman, there is no standard conversion for time.)
Dialogue: The name of the person who speaks is in all capital letters, centered on a line containing nothing else. Use one name consistently for each character, even if the character is later changed or revealed to be someone else. The words spoken by a character go from the left margin to the right, left-justified (not centered, with a ragged right margin). Turn off hyphenation.
Action: Action paragraphs are used only for actions the characters must do for the scene to work. (Anything more is telling the actor how to do their job.) Action goes inside parentheses. They are indented 1.5 inches from the left margin. (Some formats push it all the way to the center for the indent.) Do not set a tab and use it to indent each line; the smallest change will mess you up. Indent the paragraph and you can edit it without starting over on its format. In most word processing software, that's a function of Format > Paragraph > Indents.
Wrylies: You want to avoid telling the actor how to deliver a line, but if you must (if the line is shouted, for example), a one- or two-word instruction is placed on a separate line between the character name and the words he speaks, in parentheses, either centered under the capitalized character name or indented 0.5 inches from the left margin. Never use a wryly to direct an actor's facial expression.
Cover Page: Center the play's title, in capital letters, both left to right and top to bottom. In the lower right, flush with the right margin, put By and your name. Some people also add the date of completion or the start and finish dates on the next line. Do not put a copyright symbol and year.
Character and Setting Page: The characters are listed, their names in capital letters, with the briefest of bios. One line is ideal, but two is acceptable. On the lower portion of the same page, the settings, both place and time, are listed.
Header: On the pages of the actual play (but not on the cover and character/setting pages), use a header in the upper right corner, same as a novel: Surname, one-word version of the title, page number. (Smith/CLEMENTINE/6). Make sure the header starts numbering as page one, even though it will be preceded by two other pages.
Last: In the final draft which will be seen by others, when everything is complete and you will not be doing any further tweaking or tinkering, use your word processing software to keep the character's name on the same page as what he says, and to keep everything he says at one time on one page, even if this means there's a lot of white space at the bottom of some pages. This is usually a function of Format > Paragraph > Text Flow.
Really Last: If you don't know how to do some of this stuff, it's worth learning. Your word processing toolbar probably has a question mark on it. Click on it and type in what you need to know. Most of it's pretty easy. Practice on any document, then close without saving, so the document will not be affected.