Not that many- there are a couple of paths, depending upon which books you use:
1) 1 you take feats for proficiency, weap. focus and one more feat (Pole Master [PHB2] or Pole Fighter [DCv1], depending on your available sources).
(Yeah, I know that monks are feat starved...but you absolutely can't justify doing without the proficiency part. WeapFoc may be a bit of piling on, though.)
1 level in several of the warrior classes- Ftr, the OA Samurai, PsyWars, Barbarians, etc.,- and a few others like Druid or certain Clerics will get you that proficiency. Then proceed in Monk from then on.
2) OTOH, you could also take levels in the 5 level Shou Disciple PrCl, which lets you FoB with any weapon you're proficient in....and wear armor while doing it.
3) Some of the variant Base class Monks have more weapon options than the one presented in the PHB, so they may start off with a Reach weapon.
4) Some of the various sourcebooks out there add all kinds of simple, martial and exotic weapons to the game, and many of those expand the list of Monk weapons. For instance, the Longstaff in CompAdv is a monk weapon...meaning all you'd need to spend is a single feat (Exotic Weapon Proficiency). Others, like OA or Secrets of Sarlona, add several new weapons. And that's without delving into those various 3PP "armories."
5) There is a spell in the compendium that lets a weapon expand and give reach.
6) While its certainly the minority view, some (including myself) let wielders of certain oversized weapons gain the advantage of reach. That could be very good for a Half-Giant or Goliath monk...