I'll agree with ishar, but add a bit to what was said. Alot of times, on the little lady's first batch, she may only have 1 fry. Also, if she is stressed out by her environment, or her tank mates this can also cause her to have a very small batch of fry.
On the other hand, if she has had multiple batchs of fry, and is in good living conditions, you could end up with upwards of 20 fry on your hands!!!
Best of luck!! I have 20 fry in my tank, and 2 more prego females!! MAN! Is that alot of guppys!!