Ideally, you should have one that shows off every type you can play in every style you are interested in doing. I know it sounds daunting, but it's not that bad. Start with contemporary- 1 comedic and 1 tragic. Add to that Shakespeare at least 1 verse- 1 comedic and 1 tragic. Try to have at least one of the four show your youngest range and one show your older range. From there, you'll want 1 in the style of your favorite playwrights Shaw, Williams, Ibsen, Chekov, Musical Theater, etc. You want to work one in every dialect you do expertly. And you'll want at least one out of them all that is very physical and one that is very still. Most of them should be under 1 minute but you should have one piece that is 2 minutes and one that is about 30 seconds. If you are an actor of color, have at least one piece that can only be played by someone of your ethnic background and most of your pieces be for general casting. I used to have at least 9 ready to pull out on any day and it served me well as I rarely had to miss an audition because I wasn't prepared.

Buy a small notebook, large enough that each piece will fit on one page when written neatly. As part of your learning process, write your monologue in this notebook in your best penmanship, paying close attention to every word and all of the punctuation. Take this with you every day. That way if you stumble upon an extra audition you'd like to do or they ask you for an additional piece, you can give it a quick look over to make sure it's fresh in your mind. If you need help finding good pieces, see a coach.