edit 1

Script.Load("../ns2/lua/NS2Utility.lua")

local kSentryBuildRadius = 21 //WARNING: don't set to large because it will limit sentry building in adjacent rooms.
local kMaxSentryInArea = 4

local base_CheckBuildEntityRequirements //if you're a server admin you can just have "local base_CheckBuildEntityRequirements = CheckBuildEntityRequirements"
if not base_CheckBuildEntityRequirements then //This if statement is to prevent a issue with code reloads that only occurs when modders save changes to this file while ns2 is running.
    base_CheckBuildEntityRequirements = CheckBuildEntityRequirements
end

local sentryClassName = "Sentry"
local sentryErrorMessage = "Warning: Too many Sentries!"
local _EntitiesInRange = GetEntitiesForTeamWithinXZRange // Check for other Sentries on your team around the build location
local _Count = table.count
function CheckBuildEntityRequirements(techId, position, player, ignoreEntity)

    // Check if the entity is a Sentry
    if kTechId.Sentry == techId and _Count(_EntitiesInRange(sentryClassName, player:GetTeamNumber(), position, kSentryBuildRadius)) >= kMaxSentryInArea then
        return false, sentryErrorMessage //legalBuild, errorString
    end

    return base_CheckBuildEntityRequirements(techId, position, player, ignoreEntity)

end