How many recommendation letters should you ask a professor to write?

Say a student who wants to further her study in a specific field, so she applies for mulitple programs at several universities.

The question is that each of these prospective universities requires her to submit at least two recommendation letters.

The question is that how many recommendation letters should she ask a Professor to write?

If she has applied for 5 programs at 5 universities, then she needs to submit 10 recommendation letters. She knows 10 professors of her former school, in this

case, should she ask each professor to write one recommendation letter for her,

or ask the two professors she knows best to write 10 recommendation letters for her?