There are a couple of different places where this might have arisen.
1) Frequently there is a school district or campus level policy restricting how many times children are asked to actively sell something, like giftwrap or cookie dough. It's pretty common at a lot of schools. At ours, each school in our district (at least elementary) can only have two of these type fundraisers. (Note - school - not PTO.) Since our district auditor wants the school to own at least one of these, that means we can only have ONE as a PTO. However, other types of fundraisers like carnival, or school T-Shirts, or garage sales don't count against the two. Nor do the various charitable items (like Jump Rope for Heart) where people pledge. Also, Bookfair and such doesn't count. For us, it's only when we basically use the kids as a sales force for a significant fundraiser.
2) The other place this might have come from is that some states (Texas being one) only allow tax exempt organizations to have two tax exempt fundraisers per year. They can have more, but the organization must pay appropriate sales tax on the proceeds. And note - don't let paying sales tax scare you off. It's a simple form and not everything is taxable. And, you get to pick which two are tax exempt (the biggest, obviously).
You'll want to ask more questions about this "rule", but I'll bet it comes from one of these.
Another local PTO president told me the other day that PTO can not have more than two fundraisers! Has anyone else ever been told this? Is it a PTO, PTA, State Law, or what? I would appreciate any in site! (I did not ask this other local president this because i didn't want her to know we did three last year!)