Brief explanation.

You could prefix the data store keys with 'slot1_' followed by the user ID of the player.

Don't understand DataStores?

To create saving slots in a Roblox DataStore, you can use the SetAsync method of the DataStoreService class. Here's an example of how you might use it:

local DataStoreService = game:GetService("DataStoreService")
local myDataStore = DataStoreService:GetDataStore("MyDataStore")

local function saveData(player, data)
    local playerKey = tostring(player.UserId)
    myDataStore:SetAsync(playerKey, data)
end

local function loadData(player)
    local playerKey = tostring(player.UserId)
    local success, data = pcall(myDataStore.GetAsync, myDataStore, playerKey)
    if success then
        return data
    end
end