In the OTU the canonical numbers for minor non-human races are about 100 inside the Imperium and about 400 known to Imperial scientists.
But this is not the total. The actual number is more like a referee's preserve. The key sentence has guided GDW's philosophy from the beginning:
The minor races, of which there are hundreds within the area of known space, will be largely left up to individual referees, although we will be detailing some of them ourselves.
So, while 100 Imperial and 400 Charted might be correct, they are not binding numbers, and were never intended to bind.
As a more concrete example, I think that this sort of license assumes that referees are encouraged to put more minor aliens in the Spinward Marches, for example, than those 8 or so already detailed within.
The context of this statement follows (my bolding):
Aliens for Traveller said:THE TRAVELLER APPROACH
In the treatment of aliens in Traveller, several basic principles have been applied.
First, each alien race must have a developed background; this precludes the use of a random generating system (roll one die for number of legs, etc.), but ensures that an alien will be more than just a bug-eyed monster.
Second, room must be provided in the standard universe of the lmperium for referees to invent their own alien races; this is done with the concept of major and minor races. Those races which control large expanses of space and wield great power in the universe, and which are therefore likely to be widely encountered, are the major races, and will be defined and developed by GDW. The minor races, of which there are hundreds within the area of known space, will be largely left up to individual referees, although we will be detailing some of them ourselves.
Third, referees and players must be allowed to immerse themselves in the play of aliens as much or as little as they wish. This has been done by division of the major races into three groups, by degree of alien-ness, each group centered in a different area of space.