Maybe he means per day slots? (ofc, that would not account for int bonus slots)
Well sure, but in practice, it doesn't work as well as one might want against a reasonably optimized Wizard. They will always have a plethora of what are often referred to as "toolbox" spells. Spells that can answers an insane number of problems, such as polymorph. Wizards usually prepare several iterations of these, especially if their divinatiions weren't clear or if they're up against a DM like myself (I run very difficult campaigns, where a pretty high level of optimization is necessary to escape penalties of being resurrected and whatnot), or they take options like Uncanny Forethought or Mage of the Arcane Order to provide themselves some emergency open slots which can be filled at a moments notice.
It's really not that hard to become something quite close to Schrodinger's Wizard without the DM pretty arbitrarily banning options.