Yeah try not to be bothered about gaps. I end up with gaps all the time, it’s cool. The numbers are semantically absolutely meaningless.
I get myself in trouble sometimes at work by trying to design a system ‘in order’. I try to make it pretty by having my categories in the order that they occur in a process, for example, rather than in the order that I thought about them.
If this happens, sure, it’s nice. But I find myself just delaying making the structure because I’m trying to get it ‘right’. Don’t do that! Don’t worry about the numbers, their order, gaps, whatever. Far far more important is the concept of taking everything, dividing in to at most ten, and then repeating to create categories. That is the heart of this system, really. (Ref. this comment.)
If you’re working on multiple projects then you almost certainly just need too use the multiple project syntax. And in each of those projects if you don’t use many numbers, that’s totally fine. There’s no fee to use extra numbers. 