Each postmix has 3 or 4 Tx0 in it's mix. There are also 1 or 2 freeriders. If you want to know the exact number of Tx0s (including ancestors) associated with any postmix use Whirlpool stats tool to calculate it. Pre-mix are directly linked to tx0, NOT postmix - this is the Zerolink protocol.
https://video.nostr.build/fb3dc21eb3ef34f4488cc5edc983eeb658e0852deface125c866c48c23eec8e1.mp4