How many Toxic Purifier can I use on a single Toxic Planet? Is there a max to how far I can terraform my planet's mining?

until you hit Mega Rich X15, idk if it will stop at X15 or will let you go past to X16 if your at X15 but at X15<you can no longer terraform it to increase that resource production.

To clarify, you can use as many as you want on a single planet, and (although untested) is assumed to max out at X15 mega rich.

Yep that is as far as you can go but you can get it to 16X if you do it right. You would have to start on a just normal rich world and go from there or use a terraformer on a it to get that extra one before using your last purifier on it.

For an example. If you got one of your toxic worlds up to 13X mega rich you could use a terraformer to get it to 14X, then use the purifier to make it 16X I have done this so I know it works.