What numbers can you see here? 111 is perfect, 125 is ok, but the other two racers risks not being able to place in the results!
Pinning numbers so the officials and high-speed cameras can keep track of you during a race is a skill just like those for riding your bike- a good cyclist knows how to do it right!
Here is the applicable rule:
1J7(b) Riders shall place their numbers as prescribed by the Race Director or officials and in such a way that they are visible when the rider is in a racing position. When shoulder numbers are provided for cyclo-cross, they are worn on the upper arm to be visible from the front. Hip numbers shall be attached securely at least at the four corners and no accessory or hair may obstruct a clear view of the number. If shoulder or frame numbers are provided, they too shall be placed as prescribed. Numbers may not be folded, trimmed, or otherwise defaced
Let’s start with what you should not do: namely, fold, trim, or scrunch up your number because you think it will lie flatter on your back. The numbers have a big white background to make them easier for the officials and cameras to pick them out of all the sponsor logos on your jersey. Folding or cutting the white background to just show the number makes it very difficult to keep track of you in a race. Scrunching your numbers makes the number almost impossible to read in bright sunshine. Of course if you have long hair it cannot cover the number! Doing these things may cause you to miss your start or not placed in the results.
What you should do is pin the number on the side of the jersey that faces judges on the finish line (left or right side). Officials and camera can’t read your number if you pin it in the middle of your back! A good guide is to line up the lower edge of the bib number with the front seam on your jersey’s side panel; this puts the number in the ideal position when you are leaning over while riding your bike past the finish line. Use at least four pins per number and if you are pinning up a rider while they are wearing their jersey try not to stick the rider or yourself with the pins!
This short 2:33 video is a great guide: Pinning Race Numbers.