While I can't answer the question of how many patrons are involved directly, I can tell you the experiences of the past...
...at least my own. In the past, overarching themes have a cry of voices, I'd guess as many as 20-50, providing ideas, support, naysaying, critiquing, etc. Usually these are short comments about overarching ideas.
In terms of actual direct contributions, the numbers are usually smaller, from as small as 5 to as large as 20. E.G. if there is a call for NPCs or Magic items, or somesuch there will be fewer official submissions.
An even smaller number is the amount of actual pitches for adventures. I've usually seen from 3-10 (and 10 is more of an anomaly) total, full blown pitches that patrons choose between. Sometimes the votes are quite clear, and sometimes it's one or two votes that decides what is written and what isn't. (I'm still hoping for a certian KQ submission from one of the authors for a really brilliant pitch that just didn't quite make it into Halls of the Mountain King). He'd thought about writing up his pitch as more of a city ecology of sorts than an actual adventure for the magazine...but I digress.
While the pitches are being considered, and after the "winners" have been selected, there is ample opportunity for patrons to provide input. Depending on the issue (its "hotness", its controversiality, its need for design help, etc) there may be a multitude of voices or just a few. When there are many, it usually is still a disussion between a few major points (and these discussions are almost always civil and intelligent, despite being passionate at the same time).
All in all, the actual number of patrons, in my opinion, is somewhat irrelevant. A better question, for me anyway, is how much input a given patron has the opportunity to provide and likely be received. To answer that, I'd say that it's just about as much as you'd want. There's plenty of room to have your say, and while not everyone will always agree, I've never seen someone's opinion ignored/not considered.
Same goes for pitches. If you're a senior patron, you'll get the chance to pitch. There are even some senior patrons that may not pitch (they just want to be in on the design discussions, either to influence or observe). The quality of the pitch has seemed to determine the final result in the past as well. Often they are anonymous.
I guess, in short, the best answer to "how many patrons there are", at least to me, is "it doesn't matter...any patron can get as much or as little out of it as they would like."
Let me know if my ramblings were off target and you'd like me or someone else to clarify with an answer along a different vein.
