Memory Beta has this list:
http://memory-beta.wikia.com/wiki/Federation_members
Not all of those represent distinct species, of course. One assumes that any colony established by a government that is a member of the Federation is automatically also a member, unless they specifically choose not to be. But there are plenty of human (and non-human) colonies that have governments that are independent of their parent planet but are still apparently a member of the Federation. So the total of 154 would have to be counting
governments, not
worlds or
species.
Also, a fair number on that list seem to be
assumed rather than
confirmed members.
EDIT: Arse.
.