when an agent/agency requests for a certain ....

Open up Word or Google Docs or whatever you use. That should be the standard size. Send that.

It means manuscript pages. Those should be 8.5x11, 1 inch margins, double spaced, .5 inch indent with new paragraph, in Times New Roman or Courier New font.

I've always used a rule of thumb of 300 words per page. A full page may contain more, a partial page may contain less. Use that as a guideline to try to chop it off at a reasonable point.

Remember, the point is for the agent to see your writing. If they want ten pages, giving them anywhere from eight to twelve will accomplish that. And I doubt they're going to stop to count.

When I was sending out, I placed at the top of the excerpt: First ten pages (rounded to chapter break).

Like Angry Guy said, make sure you complete your scene. Don't stop in the middle just because you've reached the page limit. An extra two or three pages is better than leaving them hanging. And any agent who'd reject you for doing that isn't the kind you'd want to do bidness with anyway.

Don't stop in mid-sentence. I actually sent slightly less than ten pages a few times because the paragraph the split at the 10 to 11 page mark was long. (Think another third of a page.) Leaving an agent hanging with your pages is not a bad thing. If you feel like they'll be bored or confused if you don't send them the rest of the pages to the end of the chapter, you might want to rework the chapter.

(There are always "better" places within a chapter to break off a sample. Little cliffhangers, rising points of action, small resolutions, etc. But, in general, every sentence should be leading you to the next one - a series of hooks that keeps the reader engaged. If you feel like there are places where the story isn't doing that (especially in your sample pages) you should probably revise until those flat spots are gone.)