Say I have a spell like Victimize that says, among other things "choose two target creature cards in your graveyard"... what if I only have one creature card in my graveyard? Can I still cast the spell? Do I need to have two valid legal targets to actually cast the spell?
No, you can't. You need to specify all targets. It would say "up to two targets" if you could target less. That is also for example the downside of the card Hex.