Does the chance for finding a themed egg stack with every pet of that species that's released?

Yes and no...

As long as the pet is 42 days old or more you incress your odds of a wild egg to spawn while exploring the lost grove, however those eggs are not garentee to be themmed or to be from the same speacie as the pets you releassed. ^-^

Oh, I see! That's good to know, thank you. Does the chance increase by releasing more pets, or just one at a time?

The more you releass the higher the odds for the next egg to spawn, from what i observed, releassing 50 or so pets nearly garentee an egg to spawn in your next visit of the lost grove. ^-^

50 isn't too bad, haha! Thanks again :D

As someone that has released alot (68,708 as of right now) I can tell you that the number is fluid and you might release 300+ and get an egg spawn after 300 or you might release 50 and get an egg spawn. I would keep any release pets over 42 days old for spring fest (if release boards are a thing again) or patriotic event in july

That's a LOT of eggs... thanks for the warning!