No, 54 teams are promoted to Tercera, and the same number are relegated. Tercera has 18 different groups, 3 are relegated to Regional from each group, and the top 4 enter the promotion playoffs. From the 72 clubs that enter the playoffs, 15 get promoted to Segunda B.
Think of it as an actual pyramid, where each group has up to 4 groups in a lower tier "supporting" it. It is done to reduce travel expenses, since every group comprises an increasingly smaller geographical area. That makes it really hard to get promoted, though, since a lot of clubs are competing for a meager 3 spots in the next tier.
Ah fair enough so it's essentially like the lower tiers of the English pyramid, like Swelley said. I would have thought that, that is actually a good thing for the Spanish game, as it means the quality of team is pretty much at the same level between the 18 divisions and they are still technically in the forth tier, I don't know what that equivalent level would be, but it makes things more competitive, which is always a good thing. So do the top teams in the various divisions not get promoted as standard?
Fyi It made me chucle that you started that post with No 54