HA question

I wonder if I turn on HA I have now 2 servers the data is replicated 2 times... can I choose to have servers without and with HA?
If I would have Ceph then how this will work? data is replicated automatic 3 times?

Additionally what I do not get is 2 node setup on HA. I really wish to know why it is not recommended to run only 2 dedicated servers in 2 node HA?
What if I would connect 2 more dedicated servers it would be then 4 of them and quorum exists? or no? confused :)


Thanks!