Yes it is possible, you can spawn more than 200 Zerg creatures in a game.Of course you must have enough(lots of) resources to do this deformation.How?It is simple; at first we will mention about ,obtaining extra underling amount by just morphing your Drones into a building, here when the Drone starts to mutate, the underling limit will be decreased because there is no drone right that time so there will be underling space which you can fill with another unit.By this princible you can morph 10 drones at the beginning of the game, morphing your drone into Extractor (vespene) building which will help you an extra recon unit, at the very early stages of the game.For the late sections of the game, you can make the same process but a little big scale by morphing more than one Drones and when they are mutating into the buildings you can make Ultralisks and then sell your building mutations so you will have extra Underling limit.