You would be the best judge of the number of pictures you'll take, on average, each day, or for your full stay.
I take a about 100 pictures a day. However, a co-worker showed me his pictures from a week in WDW, and he had about 45 for the whole week. Everyone is different.
I will offer these thoughts.
I have used a digital camera for several years, and I now take as many pictures as I can.
I figure that one of them will be good (especially with kids - The best shots sometimes come when I tell them I am done taking their picture)
I don't delete them while on vacation (The screen is to small for me to see them clearly).
I bring the camera home, and move All the pictures to my PC.
My new digital camera allows me (without a flash) to take about 2 pictures per second. So if the kids are outside in light, I take about 6 pictures of each Character encounter.
If you can, I would buy another Memory Card.
I would be the largest card you can. If a large memory card is too much $, then a smaller one, for backup purposes will still give you comfortable buffer. Maybe you can borrow one from a friend or co-worker.
You don't want to spend your time, either at the parks, or when you are tired at the end of day, performing Digital Camera - Memory Card management.
I always bring a second card and Never use it. However, I always think it is better to have it and not use it, than to need it and not have it.