The canon reason is that inhabitable planets - specifically, ones you can drop humans onto where they can immediately breathe the air - are extremely rare, at a rate of a few per galaxy. Very few worlds in SA have life on them that produces oxygen. The Logicians got almost all the inhabitable worlds in the Milky Way, and everyone else got things farther out. Worlds that can be terraformed are about ten times as common, but require varying amounts of work.
For total number of colonies, there's a little bit of text in the Cargo Cults section about this, sorta. If there are 30ish civilization-sized groups (some of which are from mergers), 200ish Cargo Cults and Old-Worlder groups, and 800 unaccounted-for wormhole transits, that puts it at around 1100 original colonies. 350ish of them are failed colonies - dead worlds.
Again, all this is just the standard canon. Feel free to adjust it for your own game universe.