Hopefully not! JJ’s are reminded not to pre-number the sheet.
But as a JJ, I fully understand the urge to pre-number – that must be resisted! 
It is hard to describe the experience of seeing a rider coming, often bent over and can’t read the pinney, while a lot is going on with horse & rider. The other numbers may not be visible – on the other side, or too much horse movement to distinguish them.
Things happen fast and the numbers are only in visible range in front of the JJ for a few seconds.
Plus, if you glance down at the paper to write the number, you may miss seeing the jump!
(I have had that happen!)
Once the number(s) are actually in range to be visible, the JJ often tends to be concentrating on how horse & rider are performing, more than they are getting the number. Sometimes with distracting factors and conditions – wind blowing noisy branches, heat, cold, maybe rain.
It is crazy how easy it can be to miss seeing a number that would seem to be placed where you can’t miss it! 
Not to mention that not everyone has the best eyesight. 
Horse is galloping away, very quickly beyond number visibility – wait, what number was that ??? Well if I look at the score sheet and the last one was 35, so this one must have been 36. Right?
I call it in on the radio.
But wait – the next one is 38. What happened to 37? At that point, what happened to 36? Since I didn’t see that last number clearly.
One of the horses didn’t start? Refused out or retired earlier in the course? Maybe that last one was actually 37, not 36?
So we call it in to the best of what we know – hopefully course control has the real info and can sort out any discrepancies! But maybe course control has confusion as well.
Then it is all up to the office where scores are entered. They have the job of interpreting all of the pieces of paper they have – the score sheet, the course control sheet … etc. They may see conflicting information – so what happened with 36 and 37 at jump 14??? 36 who retired at jump 10 went on to jump 14? 37 who did the whole course never jumped 14? Hopefully all of the JJ’s wrote their cell phone numbers on their score sheet!
It can be surprising that JJ’ing can be more complicated than just ticking off each rider as they go by. Things can happen fast and the JJ doesn’t always see everything. Plus every other factor, inexperienced, distracted, adverse weather, etc.
Ideally there are two JJ’s with eyes on each and every jump. One of them has the mission to get the number above all else, the other one focuses only on horse & rider navigating the obstacle. But that’s fantasy, given that volunteers can be scarce. 
It is very, very helpful when course control updates the JJ’s on which numbers are a scratch, retirement or elim. But many don’t.
I love an announcer who is calling a few XC jumps with the rider number, that I can hear, to help me know WHO is coming my way next! And who is NOT! 