You can do this i guess ? im not sure if there will be problems if write this code

local dialogArray

if true then
    dialogArray = math.random(1, 1000000)
end

print(dialogArray)

But if i understand you right u want the npc to say any number without tracking it right ? if its the case :

local function speak()
    local dialouge = dialogArray[dialogcIncrement]
    Chat:Chat(NPChead, math.random(1, 1000000))
end