I've pondered this question myself.
Every single Jaffa requires a Goa'uld symbiote to live, and they even go through several in their life, so the Goa'uld should outnumber the Jaffa, shouldn't they?
And even though they use the symbiotes to enslave the Jaffa, why would they want to grow more competition anyway? The Goa'uld are a greedy and selfish race.
If I had to guess, I'd say that either only a very small number get implanted as the Goa'uld please, with the rest being discarded somehow (we did see in Summit they were eating them!), or the competition among the Goa'uld is just so fierce that only a few are able to survive long once taking a host.
Or maybe they do survive, but we simply don't see them, because the only way they can survive is by hiding away from the more powerful Goa'uld and building their forces. We do only see a small number of the Goa'uld, but we did occasionally see there were plenty more of much lesser importance, so obviously they wouldn't get shown a lot.
But the ratio certainly does seem off for a race that works under the pretense of being invincible gods! :lol