Still need a command for this... only need to know when 1 person remains on a team. I read somewhere about a TeamCount thing but it wasn't for 1.13 and it seemed complicated. There must be an easier way for this right?
Assuming you have a scoreboard called "teams" and a team: Blue
scoreboard players set Blue teams 0
execute as @a[team=Blue] run scoreboard players add Blue teams 1
execute if score Blue matches 1 run YOUR COMMAND
These commands should be run every tick.
This setup basically counts all the players in your team every tick and if the sum is exactly 1, then the command is executed.
Command #1: set the score for Blue to 0. Notice that this is NOT the team, but just an entry in the scoreboard. Some people like to call it a "fake player". It could have any name.
Command #2: find all the players on team Blue and for each player, add 1 to the score of Blue. The first Blue here does refer to the team name. It should be exactly the team name. The second Blue refers to the scoreboard entry that was mentioned in the first command. These names should match up.
Command #3: test if the score for Blue is exactly 1 and run a desired command if true. This Blue refers to the scoreboard entry.