What comes after 15ma in Piano music? I have tried searching it up, but nothing helpful comes up.
Well, purely theoretically, a three-octave transposition would be 22ma. A four-octave transposition would be 29ma. You just keep adding 7 to each previous number.
However, don't use these. No one except true music nerds will be able to puzzle out what you mean. It's not standard music notation. I've never seen an example of this, and even 15ma is exceptionally rare, to the point that many musicians wouldn't know what it means. (However, an internet search tells me, weirdly, that some music fonts include glyphs for 22ma or even 22mb!)
For piano, either use ledger lines or change clef instead.
The only utility I can think of potentially for these symbols might be in inserting cue notes for an instrument in a wildly different range and refusing to change clef. (But cue notes frequently don't notate correct octave anyway, so this sort of thing simply isn't necessary.)