The Literal Commandment
Whatever I command you, you shall be careful to do; you shall not add to nor take away from it. (Deuteronomy 12:32)
This tells us that we should not change the number of commandments (neither add nor take away) but it also says we should not change the scope of any commandment. For example, while we are commanded to bind tefillin upon the head, we should not bind two tefillin upon the head because that would be expanding the scope of the commandment.