Why won't any of my stats save? They stick at 1 value and don't save.

You're trying to set child[i]'s value to a function that doesn't return a value on line 22... this doesn't make any sense. Just use the SetAsync function without trying to set the value of child[i].

Also, on line 15, you're setting the value when a player joins directly to the GetAsync function.. the GetAsync function can return nil, if this is the case then your code will error. This can happen if the player has never been to your game before, and they have no saved stats. So check if there are saved stats before setting them.

local ds = game:GetService("DataStoreService"):GetDataStore("stats")
local stats={"Intelligence","MaxEXP","EXP","MagicEnergy","MaxMagicEnergy","Level","LastLevel","Defense","Strength","Team","Cash","Wind","Fire","Earth","Water","Reputation"}

game.Players.PlayerAdded:connect(function(plyr)
    local a=Instance.new("NumberValue",plyr)
    a.Name="leaderstats"
    for i=1,#stats do
        local stat=Instance.new("NumberValue",a)
        stat.Name=stats[i]
        local data = ds:GetAsync(plyr.userId..stats[i])
        if data then    
            stat.Value = data
        else
            stat.Value = 0
        end
    end
end)

game.Players.PlayerRemoving:connect(function(plyr)
    local child=plyr.leaderstats:GetChildren()
    for i=1, #child do
        ds:SetAsync(plyr.userId..child[i].Name,child[i].Value)
    end
end)