How can I make a team captain selecting script function at the start of a game?

You can use game.Players.NumPlayers to find the number of players in the game, or use #game.Players:GetPlayers() Check for four players and then start the script.

repeat wait() until game.Players.NumPlayers >= 4 -- Here is the code I added to check for four players.

wait(10) 
local PESmembers = game.Teams:WaitForChild("PES"):GetPlayers() 
local FIFAmembers = game.Teams:WaitForChild("FIFA"):GetPlayers() 
local tool = game.ReplicatedStorage.CrestSelection

local PEScaptain = PESmembers[math.random(#PESmembers)] 
wait(1) 
print(PEScaptain.Name.." has been selected as the PES captain!!") 

local FIFAcaptain = FIFAmembers[math.random(#FIFAmembers)] 
wait(1) 
print(PEScaptain.Name.." has been selected as the FIFA captain!!")

game.ReplicatedStorage["Select"]:Clone().Parent = PEScaptian.PlayerGui 
wait(30) 
game.ReplicatedStorage["Select"]:Clone().Parent = FIFAcaptian.PlayerGui 

Accept the answer and upvote it if I helped! :)