How many Pops can it hold if i count the "production" against the death by age?

varies, the happiness level has an impact since it has an impact on life expectancy.
the longer any beaver lives the higher the population a single pod supports.
But if I'm remembering correctly, 15 to 18 beavers per pod.

It seems to start at approx. 12 beavers per pod, quickly jumps to 15 per pod (once you build barracks for everyone), can be extended up to 18 with certain monument.

Thx helps to make sure my Beavers grow as much as i like and not uncontrolled