6 Players on one Team?

If you are only allowing 12 people into the game at once, then ROBLOX's default Team sorting will not allow for more than 6 players on any team, so long as the number of players in the server never actually gets above 12.

That said, you have to loop through the Players of a game to get how many are in each Team, as Teams are purely organizational: they don't have any methods of their own, regardless of how useful that may be (which is why most developers will design their own Team system, rather than use ROBLOX's).

local TeamColor1 = BrickColor.new("Bright red")
local TeamColor2 = BrickColor.new("Bright blue") --Change these two to reflect your game's TeamColors.

function GetTeams()
    local team1, team2 = {}, {} 
    for _, v in ipairs(Game.Players:GetPlayers()) do
        if v.TeamColor == TeamColor1 then
            table.insert(team1, v)
        elseif v.TeamColor == TeamColor2 then
            table.insert(team2, v)
        else
            error(v.Name .. "'s TeamColor does not match either provided TeamColor.")
        end
    end
    return team1, team2
end

Game.Players.PlayerAdded:connect(function(Player)
    local team1, team2 = GetTeams()
    if #team1 <= #team2 then
        Player.TeamColor = TeamColor1
    else
        Player.TeamColor = TeamColor2
    end
end)

teams = game.GetService("Teams")
team1 = teams.Team1 -- just for example, this will be the "Really red" team
team2 = teams.Team2 -- and this the "Really blue"

game.Players.PlayerAdded:connect(function(player) -- when a player enters
    team1players = team1:GetChildren() -- get the players in that team
    team2players = team2:GetChildren() -- same
    if #team1players > #team2players then -- if team1 has more than team2
        player.TeamColor = BrickColor.new("Really blue") -- the player goes to team2
    elseif #team1players < #team2players then -- if team2 has more than team1
        player.TeamColor = BrickColor.new("Really red") -- the player goes to team1
    else -- the teams are equal
        player.TeamColor = BrickColor.new("Really blue") -- the player goes to team2, or team1 (which one you want, just because they're already balanced)
    end
end)

Observation: players have the TeamColor property, teams are separated in colors.