I would say as many as you start with in your game. Mostly because games fluctuate. You are starting your game therefore you will need all the info you can produce to make it effective. If half your players drop 2 weeks in then so be it. You can still recruit for more. But I would definitely go with how many you have accepted as players when you start the first thread. If you have 20 people who sent in character sheets ready to play then you would get 40 threads. This is far more than you'd likely ever need. I've never had to use more than 5 threads to start. And I wouldn't make a ton of IC threads. I find that when you have more than 3 IC threads people get too spread out and lost in the mix and you can't keep track of who is where and others don't want to go searching through tons of RP threads to find out whats going on or who is doing what. Thats just my experience and insight, mind you. Given most small groups that start off they typically have 6-10 players. That gives you 12-20 threads to work with. More than enough.
That is what I thought.
I've never come close to the limit. The nearest was with a small group of six players where I had an OOC thread, a character thread, a NPC thread, two IC threads for real world and one IC thread for cyberspace. That's half the number allowed.
However, lets assume you have a sandbox game that starts with (say) six players - okay so twelve threads:
1) OOC discussion
2) Approved Characers
4) Background info
5) Story summary
6) List of locations and descriptions
That leaves six threads available for IC location threads. More than enough? Maybe, until people start wanting private threads to go off and pair up in...
Point being, it's worth considering what non-IC threads and information threads you actually need before you start, and then trimming them as appropriate based on the number of players.
I make it a point, these days, when opening a new OOC thread or recruitment thread to reserve the first few posts for my own use, just by posting the word "reserved". This gives me the option should I need to of going back later and adding info to that post. It means that the initial post can stay relatively compact with the information I placed there to attract players and not expand into unreadability.
Things like my in-house rules, character sheet templates, thread links, post header templates, background info, location descriptions, all these can sit in the reserved posts.