On the back of every game it states how much KBs/MBs it requires on a memory card. How much space is on the memory card is either stated on the card itself or it's packaging. An average memory card for ps2 is 8 MB.
I think I have close to 25 saves, with room for more.
Recent got Final Fantasy X, haven't played it yet, I'm trying to like the turned based games because I know I'm missing out on a lot. I'm also considering looking into God of War II.