Ok, a couple of factors here.
Let's say, hypothetically, Xbox has 6 big games to announce. (Let's say Halo Infinite, Hellblade 2, Fable(?), New Forza, and maybe a couple unannounced surprises).
They need to at the very least match Sony's latest reveal of big games (So that's Horizon 2, Ratchet and Clank, Spider-Man: MM, Returnal, Project Athia, Gran Turismo, Demon Souls Remake, Sackboy)
Mind you I'm only counting big games of AAA caliber. When I say "match" it doesn't necessarily mean quantity, but more like quality.
If Microsoft has less games, they would need a Killer App in order to hold back the reveal of other games. Killer Apps are rare by definition.
At the same time, they need to be mindful that Sony more than likely has other games they have not revealed yet and in a sense this means they cannot afford to put all their eggs in the July 23rd basket. They need something to hit back with.