i think the real question is which tax returns does he have. there are many entities and organizations he's associated with.

he (sdny, at a minimum) surely have everything related to the inauguration fund. i strongly suspect he (and/or sdny) has his "charitable" foundation returns.

not as clear if he's got other things, like trump organization or donnie's personal returns, but it's certainly possible.

I’ve heard several prosecutors say they probably have them.

All depends on Trump's behavior or lack there of.

Taking over the case from the IRS was probably the 2nd thing Mueller did, right after locating the restroom in his new office.

that he is investigating. If he has evidence that Trump cheated on his taxes because his other knowledge shows that, he might give it to the IRS. If they just show he lied to the public, that wouldn't be enough for him to give out information from them. Only if they show reasons why he conspired with Russia will him having them be relevant. I don't think he'll hand them over to Congress just because they ask; he'd say that any subpoena for them should be directed to Trump himself.