When I say 15, I include those due back from loan.
Even then, impossible. There aren't 4 of them on loan good enough. Maffeo would be an obvious choice, but if we're going after Sanchez and Mbappé then Unal, Celina, Roberts, Zinchenko - all the forwards are ruled out.
Mooy isn't close to being good enough, Moreno needs at least 1 more year, he's barely got his feet on the ground in Spain.