Some new player questions

hi, couldnt figure out some things that might seem obvious
wondering if you could help me out
-how many units can you bring into battle? I thought it was 6/6, but i just bought one and now I can bring all 7 lol how does matchmaking work, does it match people based on # of units, or AP, or what?
-as a new player, should I be saving up for higher level units or buy some now to expand my team # of units? I know there isn't a right way, but what is generally advisable on how to spend ducats now (I am part of faction of Light/lion)
-What is AP exactly?
-How does a unit die? If your unit gets eliminated during battle, does it 100% mean it will die? (need to use fate card to revive, right) What is the point of injury% in missions, units dont come back injured right? does it have a chance of dying during a mission?
-How does experience gain work? do some units gain more than others, and is a total exp split between all units (meaning using less units = more exp for them)
Thanks in advance!

-how many units can you bring into battle? I thought it was 6/6, but i just bought one and now I can bring all 7 lol how does matchmaking work, does it match people based on # of units, or AP, or what?
-as a new player, should I be saving up for higher level units or buy some now to expand my team # of units? I know there isn't a right way, but what is generally advisable on how to spend ducats now (I am part of faction of Light/lion)
-What is AP exactly?
-How does a unit die? If your unit gets eliminated during battle, does it 100% mean it will die? (need to use fate card to revive, right) What is the point of injury% in missions, units dont come back injured right? does it have a chance of dying during a mission?
-How does experience gain work? do some units gain more than others, and is a total exp split between all units (meaning using less units = more exp for them)
Thanks in advance!