How do I set a max value for a numbervalue?

There isn't any property of NumberValue that let's you do this, but remember that there's an event called Changed for this object that fires whenever its value is changed. So with this is mind, simply connect a function to the event that checks if the new value is above the maximum, and if it is, lower it to the max.

local max = 100

local numberValue = Instance.new("NumberValue")
numberValue.Parent = workspace

numberValue.Changed:Connect(function(value)
    if value > max then
        numberValue.Value = max
    end
end)