Any reasonable official interprets that rule to mean you can’t take your issued number of “121” and trim it to “12” or “21.” Purpose of the sticker is to help with identification and trimming it as such hinders that.
On the other hand, if you’re issued a race number with several inches of “extra” space on the top, bottom and sides, it’s perfectly reasonable to trim it. In all my years I’ve always trimmed my down and only been challenged once. It was pre-race and I protested and insisted that we go talk to the head official. We did and he looked at the other official, rolled his eyes, and sided with me.