There is no special rule abut number of spells per round outside of their action costs.
1 action spells (like shield and true strike) are fairly rare, but the Quicken spell metamagic feat would allow any 2 action spell to be a 1 action spell (once per day).
Yes, you could even cast three 1 action spells if you don't do literally anything else with your turn.
4 if you have any reaction spells.
So, for example, I could cast Guidance (1 action cantrip) three times per round?
Obviously, each target can only receive the benefits once per hour (per the cantrip description), but if I am part of a large party, then I could put Guidance on 6 characters in the first two rounds, right?
Yup! The 1 hour immunity is the only limiting factor in this case, like you said.
This means yes, 1-action spells are much like Quickened spells in PF1. They might be similarly "4 levels weaker" (whatever that translates to when brought into PF2), but they're still very useful.
Many of the mediocre Focus Spells are 1-action spells, meaning they're actually great since you can cast them alongside a 2-action spell for a powerful round of casting.
It is possible to cast two two action spells in a round, but these are once per day.
Spend one action to tell your Familiar to do it instead.