|Creature||Timespan||Time to maturity||Lifespan||Min number||Max number||Average number|
To get the minimum number of ancestors for each row we divided the time in years by the lifespan. To get the largest numbers of ancestors we divded the time in years by the time to maturity.
This gave grand totals of between $19,658,333$ and $154,576,923$ ancestors.
To get the average age of an ancestor when giving birth we found an average age = (Lifespan + time to maturity)/2 and timesed this by the timespan.
Assuming that ancestors were born at an average point in their parents' lives, we get $33,988,937$ ancestors.
Rounding up to sensible values, we think that there were at most $150$ million ancestors, at least $20$ million ancestors and probably about $35$ million ancestors.