As you guessed... it depends. A 4 megapixel camera, can take pictures that are very high resolution (very big), to "ok" resolution "very small".
1 megapixel is 1 million pixels (dots) on the actual picture
2 mega is 2 mill
3 mega is 3 mill
4 is 4
yada, yada, yada
a 4 megapixel picture is 1,704 X 2,272 which in turn is 3.9 million pixels AKA 4 megapixels (close enough to say 4 million)
more pixels, means smoother lines, more clarity, etc. It has NOTHING to do with the printed SIZE. It will either use 1 million pixels to print a picture on a 4 X 6 or 4 million pixels to print on 4 X 6.
A read that a 8 X 10 picture needs 5.2 million pixels to print on every spot on the paper. Would you use a picture that has 1 million pixels or 4 million pixels. In otherwords, if you plan to blow up pictures, you need a higher mega pixel camera. No intent... no need.
Back to your original question. How many on a 64 meg stick. Lets assume you are using a 2 megapixel camera. The highest quality picture that a 2 megapixel could take is 2,160,000 pixels which is pretty much 2 megabytes in size. If you have a 64 meg stick, it can hold about 30 to 32 depending on the amount of overhead required to store pointers and such. Now the same 2 megapixel camera, can also take lower resolution pictures.
Say instead of 1200 X 1800, you took 288 X 432, in that case, you require only .125 megabytes per picture. Low quality, but if all you are going to do is put it on the webpage... it probably is enough. With this, you can store a couple hundred pictures on a 64 meg stick.
Confused enough yet? Did I even answer the question? LOL
Please go ahead and ask more and I will try and clarify the areas. This is confusing stuff.