Just started prozac 20mg 6 days ago. was taking 10 mg of lexapro (escitalopram) before. he many mg of prozac would equal the same as 10 mg lexapro (escitalopram)?

It's not like that: You're comparing apples and oranges. Different medications with different mechanisms don't have equivalent doses. Most medications have a usual starting dose which has been shown to have reasonable efficacy and tolerability. Then the dose is adjusted according to clinical response. You can't say, "This much of this one is the same as that much of that one." It doesn't work that way.