Yeah, that likely is technically true. PTA has roughly 5.5 million members these days.
Huge difference, though, in how the PTA gets its members, as opposed to those other organizations.
I'd venture to say that all or nearly all of the members of the Rotary Club join that club understanding the commitment they are making to the club (meeting attendance, service, etc.) and there's also often a trial period/application/approval process for entering those clubs. Safe to say that a very high percentage of the members of the Rotary are very involved with Rotary, could identify Rotary's mission, etc.
On the other hand, the vast majority of PTA members pay $4 or $10 to be a member of the parent group of their kids' school. It's almost rote. In a group of 240 members (the average PTA unit-size), maybe 10-20 are highly involved, another 10-30 help occasionally... and 200 or so of the members only connect by paying that $10 or eating some ice cream at a PTA social.
There's nothing wrong with that (it's exactly the same for PTOs), but comparing Kiwanis membership numbers to PTA membership numbers is like comparing apples and bicycles.
There is no exact measurement of PTO numbers, because: 1) PTOs are independent and don't have to report in globally to a national group; and 2) many PTOs don't charge dues and include "all parents at XYZ school" as members (I suppose that could count as 800 members, if it mattered.). That said, if measured the same way as PTA measures ("number of parents who belong to a PTO at their local school"), my best conservative guess would be:
Roughly 55,000-60,000 PTOs.
Roughly 175-250 members per PTO.
Total = 11 million plus PTOers.
