Complex sentence: While there are tens of thousands of Goa'uld across the galaxy, only dozens hold the rank of System Lord, most of which are minor System Lords apparently excluded from high-level decisions, as shown in the Season 5 episode "Summit".
Split into simpler sentences: While there are tens of thousands of Goa'uld across the galaxy, only a dozen or so hold the rank of System Lord. Most of these are minor System Lords apparently excluded from high-level decisions, as shown in the Season 5 episode "Summit".