I think your question was how many subforms can be inserted on a form, not how many times a single subform can be put on a form. However, I don’t have an answer. I had a complicated form that I tried to turn into abouit 15 suforms on tabbed tables. After about 10, the form would not load. However, I think it is a factor of the number of fields on the subforms as well.
The best thing to do would be to test for your applcation and see how it works out.