>>5575114212 at the beginning with possibly one or two more for legendaries in a possible DLC or as a final boss
The question is "will they give it to Pokemon who deserve it? or will it just be another popularity contest to sell merchandise?"
For example, on the one hand I feel that they owe it to Delphox and Chesnaught due to the blatant favoritism to Greninja and how in future generations it was more equitable (moves-Z, Gigantamax) but if this Legends ends up using other Starters like in Hisui, these will be the ones receive the Mega form.
Another possible case is that we end up with 9 mega forms for Kantomons and 3 random pokemon from other generations.