How would I find the amount of players in a team?

The NumPlayers property is only found under the service Players. To find the number of people on a teams you may have to do a comparison check with the Players TeamColor and the Teams TeamColor.

local function getPlayersOnTeam(tColor)
local plrs = {}
for _, player in pairs(game.Players:getPlayers()) do
if (not player.Neutral) and player.TeamColor == BrickColor.new(tColor) then
table.insert(plrs, player)
end
end
return plrs
end

print(#getPlayersOnTeam(game:GetService("Teams")["Playing"].TeamColor))

If you just want to call the function with only TeamColor, you can do:

local function getPlayersOnTeam(tColor)
local plrs = {}
for _, player in pairs(game.Players:getPlayers()) do
if (not player.Neutral) and player.TeamColor == BrickColor.new(tColor) then
table.insert(plrs, player)
end
end
return plrs
end

local tabOfPlayers = getPlayersOnTeam("Bright Red")
print(#tabOfPlayers) -- Prints number of people on team

Checking For One Player

As per your comment, you are interested in finding one Player it seems. Since we have a table of Players on one team, it's easy to do this!

local function getPlayersOnTeam(tColor)
local plrs = {}
for _, player in pairs(game.Players:getPlayers()) do
if (not player.Neutral) and player.TeamColor == BrickColor.new(tColor) then
table.insert(plrs, player)
end
end
return plrs
end
local num;
local team = game:GetService("Teams")["Playing"];
repeat
local num = (#getPlayersOnTeam(team.TeamColor))
until num ==1 
local plrs = getPlayersOnTeam(team.TeamColor)
if plrs[1]:FindFirstChild("leaderstats") then
plrs[1].leaderstats.points.Value = plrs[1].leaderstats.points.Value + 1
end

Accessing everyone's playerGUI & TextLabel

As per your comment, you are interested in accessing everyones PlayerGui. Believe it or not, this can easily be done with a loop:

for _,player in pairs(game.Players:GetPlayers()) do
if player.PlayerGui:FindFirstChild("Intermission") then
player.PlayerGui["Intermission"].TextLabel.Text = "Intermission Started!"
end
end

Code Above Is Untested