Most of those 4359 are inactive. I don't know a way to tell how many different people have posted in the last month, for example. Some of those on that map are also inactive and not everyone currently active have pinned themselves.
That's an interesting question. How do you consider if someone is active or not? There are people like Hoodat, Secuono, or maybe Monty or Dew that occasionally drop in, but aren't on here on a daily basis, or even weekly or maybe monthly but I still consider them a part of the group. It's hard to say they are active though.
I went to the "Recent Posts" tab at the top of the page, went back 10 pagess, and counted the number of names that show up. That only shows the last people that posted on that thread not everyone that has posted on the thread, but I came up with 44 people. I expect @Nifty could come up with a very accurate number if he wishes, but he'd also have to have some kind of criteria as to what active is. Maybe there is some industry definition.
For what it is worth, there are 75 users online right now, 5 members, 50 unregistered guests, and 20 robots. Do you consider the 50 unregistered guests active? I'm sure several guests are using the forum regularly but aren't talking to us.
But looking at the last 10 pages I'd guess around 40 to 50 really active with several more occasionally here.