So what does a prosecutor need to show the judge to obtain a tax return? The statute requires the prosecutor to show three things.
First, the prosecutor has to show "there is reasonable cause to believe" that a specific criminal act has been committed. That's where the recent news of the Manafort search warrant comes in. We know that Mueller had enough evidence to establish "reasonable cause" that a crime was committed because he got a search warrant.
Mueller would also have to establish that he could not "reasonably" obtain the tax return from another source, like the person's accountant. Generally speaking, that is an easy thing for a prosecutor to prove. If they can't prove that, it means they will just get the tax return elsewhere.
Finally, Mueller would have to establish that "there is reasonable cause to believe" the return "may be relevant" to his investigation. That is a very low bar, much lower than what Mueller had to establish to obtain a search warrant for Manafort's home. To obtain the search warrant, Mueller had to establish that there was probable cause to believe there was specific evidence of a crime at a specific location — Manafort's home.
All that he has to show is that the tax return might help him in his investigation. That means that even the tax return of someone other than Manafort would be helpful to Mueller as he conduct his investigation. At this point, we don't know what other evidence Mueller has and exactly whom he is focusing his investigation upon. But given that all he has to show is that the tax return "may be relevant" to his investigation, which has a very broad mandate, he could have tax return information for many individuals.
That brings us to the obvious question — does Mueller have the president's tax returns? There is not enough public information for us to know. Public reports of subpoenas relating to Trump Organization financial records could suggest that Mueller may have sought tax return information of those business entities, if not the president himself.