How do I make a script that detects how many players are in an Area?

For example, if there were 3 players in said area, a NumberValue's value would be set to 3. If there was 20 players, then the value would be set to 20.

Here's what I have so far (in a normal script):

local NumberOfPlayers = script.PlayerCount
local PlayerCount = NumberOfPlayers.Value

function AddPlayer()
	NumberOfPlayers.Value = NumberOfPlayers.Value + 1
end

Other note: the script is within a part, so the ancestry looks like PlayerCountBlock.CountingScript.NumberValue