I would purchase just one cloth diaper, one plastic pant and some diaper pins and see if you like it.
If you don't like it, you haven't wasted money.
If you do like it, then the number to get will depend on how often you wear cloth diapers and how often you are able to wash them and whether you will wash your diapers with other laundry or by themselves.
Number of inners used in a day X number of days between washes, + 2 inners just in case is a good rule of thumb. For example, 4 inners a day, X every 2 days = 8 + 2 = 10
If you go with AIOs (all in ones), everything is one and done, but drying time suffers. The concept I like best, and will probably use when I sew mine, is the AI2, where the absorbent inner snaps into the PUL cover. I like this concept because I could get by on 10 to 14 inners, and 5 to 7 covers. Just rinse and reuse the cover. I might rig mine so I can snap several inners atop one another to boost absorbency. Also, if desired, it's possible to do the prefold and cover method with the same covers you already use in the AI2 system.