Acts generally break down the dramatic structure of a play. David Mamet discusses the 3-act structure in his book Three Uses of the Knife. I'm sure you've heard about Shakespeare's 5-act structure, too -- Set-up, Rising Action, Climax, Falling Action, Denouement (I don't agree with it it all, but there you are.)
An act can have as many or as few scenes as are necessary. In The Merchant of Venice, for example, Act II has 9 scenes. Act V has 1. It really depends on breaks in time or place. "French scenes" change every time a character enters or exits (as in Moliere's plays). Then again, some plays play with time and space within one scene, or one act.
Some plays are written just as scenes (and do not expect an intermission). Some are just acts. It really depends on the writer and the play. There are no hard rules.
Welcome aboard. But I would suggest you get a good book from the library to educate yourself in the basic terminology of theatre and playwriting. Maybe your teacher can guide you in the right direction. Your library should have many copies of books about theatre.
The one I most admire is "The Elements of Playwriting" by Louis Catron, but there are many others just as good and maybe some members would add to this list of books for you.
Here on this site, you will be a getting concise, but brief Cliff's Notes versions in answer to your very good questions, but they will never suffice in depth for what you will need in your quest of becoming a playwright.
Will others post playwriting book recommendations?
i once wrote a play in twelve acts. i think preston sturgis was the one who started this structure.
you have a set up. then something happens that changes things. and the lead character must decide what to do. that's the end of the first act.
you keep doing this. one complication after the other. now what to do?
it keeps the pace moving along. but you must keep in mind what you're trying to say all along. you can't just make it an exercise in conflict. in the end, the lead character must become a different person. we're just along for the journey.
At this point, the best thing for you to do is to go to http://www.stageplays.com/browse-no-frames.cgi?group=playw and pick out a book or, better yet, one of the books that others have suggested and order it. This is the absolute best advice I know to give you from having determined what stage you are on in your journey. If you were my brother I would advise you to do this before taking another step.