Animate Dead (Zombie/Skeleton) or Create Undead (Ghoul) do not count as followers and theres no limit mentioned anywhere except the number of spell slots available.
Also the spell specifically mentions maintaining control over 4/3 undead per slot so limit can't be 2.
Numbers get pretty insane at higher levels that said the value of 100 zombies is limited for a lvl 13+ wizard.
https://roll20.net/compendium/dnd5e/Animate%20Dead#content http://dnd5e.wikidot.com/wizard:necromancy
As for higher level summons, Animate Object is pretty powerful but its only temporary and you can have only one as its concentration (summon monster is concentration as well now):
https://roll20.net/compendium/dnd5e/Animate%20Objects#content
