How do I count players in a team?

It may be satisfactory to just get the players on each team.

We just filter all of the players by matching their TeamColor to a Team's:

function getTeam(team)
    local players = game.Players:GetPlayers();
    local teamPlayers = {};
    for _, player in pairs(players) do
        if player.TeamColor == team.TeamColor then
            table.insert(teamPlayers, player);
        end
    end
    return teamPlayers;
end

For the actual moment of entering and leaving, we can listen in to PlayerLeaving and PlayerAdded.

function PlayerLeftTeam(player,teamcolor)
    -- Do whatever
end

function PlayerLeaving(player)
    PlayerLeftTeam(player, player.TeamColor);
end

game.Players.PlayerLeaving:connect(PlayerLeaving);

(and the same for PlayerAdded)

To add on to Blue's response, that function wouldn't fire when the player switches teams. So this is what I'd do.

function PlayerLeftTeam(plr, team)
    print(plr.Name .. " left " .. team.Name .. " team.")
    -- code
end

function PlayerAdded(plr)
    local lastTeam = plr.TeamColor
    plr.Changed:connect(function(p)
        if p == "TeamColor" then
            PlayerLeftTeam(plr, lastTeam)
            lastTeam = plr.TeamColor
        end
    end)
end

function PlayerLeaving(plr)
    PlayerLeftTeam(plr, plr.TeamColor)
end

Game.Players.PlayerAdded:connect(PlayerAdded)
Game.Players.PlayerLeaving:connect(PlayerLeaving)