Is thee a general rule on how many hosts shold be reserved for HA?
Is N+1 the general rule based on how many nodes?
when do I need +2 or 3? and should the +1,2,3 be in maintenance mode?
There's no general rule of thumb for this. It actually depends on the individual needs and requirements. Consider an environment with 2 sites, where the hosts on one side need to have sufficient resources to take over if the other side goes down. Unless the hosts have spare capacity, you may even need to configure N+N in such a case.