Primus created 13 Transformers... not sure if all of them were "Primes" (don't know if you'd consider the Liege Maximo and Maccadam to be Primes). In fact, in G1 continuity, being a "Prime" was merely a titled given to Autobot leaders. It wasn't a "genetic dynasty" like in the movie continuity. And even then, according to the movie continuity there were only 7 Primes - so presumably the other 6 original Transformers weren't Primes... which would make sense, because if all the original Transformers were Primes, then wouldn't it mean that all the current Transformers would be their descendants (and not just Sentinel and Optimus)? e.g. Megatron, Bumblebee, Ironhide et al. are most likely descendants of those other 6, since none of them bear the "mark of Prime." But again, all this stuff about the dynasty of the Primes comes from the movie universe - AFAIK this concept isn't a multiversal singularity, so doesn't necessarily apply to other continuities.