You can see this collector number on the bottom of each card, usually followed by a slash and the total number of cards in the set, excluding secret rares
There are no restrictions on this, so theoretically one could use a Tinkhec on a Tinkhecitd make for a pretty huge monster, considering Tinkhec starts out at 2700 ATK
Main Phase You can play cards, and attack with your Leader or Characters
Ankle Shanker Bane of the Living Beacon of Unrest Blood Tyrant Consuming Aberration Corpsejack Menace Crackling Doom Cruel Entertainment Curse of Vengeance Curtains' Call Duneblast Everlasting Torment Glint-Eye Nephilim Guiltfeeder In Garruk's Wake Languish Lavalanche Magus of the Will Merciless Eviction Nath of the Gilt-Leaf Necroplasm Reyhan, Last of the Abzan Sangromancer Sphinx Summoner Tymna the Weaver Utter End Vulturous Zombie Waste Not Whispering Madness Worm Harvest
A few standout choices include Kenrith, the Returned King, Agrus Kos, Eternal Soldier, Tazri, Stalwart Survivor, and Dynaheir, Invoker Adept