No, i tested it. You get one spawn more of each creep-categorie..
So from: 7 Minos, 3 Archers, 2 Priests, 2 Catapults, 3 Angels, 1 Fattie Golem => 8 / 4 / 3 / 3 / 4 / 2 Upgrading your spawns from 18 => 24.
Well, i'd be careful with tapping the flag to early. It help the other team instead of harming them. Specially since the starting creatures are one-hits anyway. Because you spawn them more, they'll get more XP + Gold. I wouldn't tap it until the Catapults spawn or maybe the Priests.
So long,
Aspartem