How do you get the highest number out of a leaderboard?

players = game:GetService("Players")
highestValue = nil

for i, v in pairs(players:GetChildren()) do
    if v:FindFirstChild("leaderstats") ~= nil then
        for ii, vv in pairs(v.leaderstats:GetChildren()) do
            if highestValue == nil or vv.Value > highestValue then
                highestValue = vv.Value
            end
        end
    end
end

That should find the leaderstat with the highest's Value and give you that number

This I'm not 100% about, but it should give you the highest leaderstat instance and that stat's Value. It may take some experimenting if it doesn't work, but this is the general idea that I would use.

players = game:GetService("Players")
highestValue = nil
highestStat = {nil, 0]

for i, v in pairs(players:GetChildren()) do
    if v:FindFirstChild("leaderstats") ~= nil then
        for ii, vv in pairs(v.leaderstats:GetChildren()) do
            if highestValue == nil or vv.Value > highestValue then
                highestValue = vv.Value
                if highestValue > highestStat[2] or highestStat[1] == nil then
                    highestStat[1] = vv.Name
                    highestStat[2] = highestValue
                end
            end
        end
    end
end