You can have as many POV characters as you like, either in first or third (within reason, of course; if you have 30 different POVs in one book, your reader is probably going to get confused). I've done a mixture: one ms switches between two POV characters; its prequel switches between four POVs; another ms switches between three; and yet another, like the first, switches between two POVs, though we also have a switch in time between the two POVs of a hundred years. A novella is strictly one POV. Those are all third person. One ms is first person and only one POV.
The ms I just finished is all first-person, told from four POVs and from the perspective of different years. Interestingly, the characters are literally telling their stories to someone, and they are actually already dead, to speak to Sarahrizz's comment above. It's not too common to see this kinn of framing device anymore, but in the 19th century, most novels were framed as a story being told (verbally or by found letters or what have you--hence the story-within-a-story-within-a-story structure of Frankenstein). Today, we don't actually expect that the narrator is literally telling the story or writing the story down; we're simply privy to their thoughts/memories, all of which could be going through their mind in the split second before they snuff it.
As for the other question about alternating POVs--no, there need not be any pattern. Use whichever POV works best for each scene/chapter. Maybe that character's voice will put the situation in an interesting light; maybe you need that POV character because they see/do something important. Sticking strictly to a pattern will force you into adding unnecessary scenes/chapters just to keep the pattern going.