The "missing" 6 is usually caused by either a page deletion or a sheet reordering. Nothing to worry about.
I was assuming (bad to do some times) that there was no inherent problem with the gap in sheet numbers. It would be nice to eliminate this, as it would be easier to find unnamed nets or components with no refdes if the generic instances corresponded direclty to sheet numbers.
Is there any way to resequence these instances?
BTW, I'm using 9.3
The $n notation defines the unique ID of a symbol so this is fixed once the symbol is placed. You cannot re-sequence such ID's as doing so will cause issues with downstream tools such as PCB layout etc. Other than having an 'out of sequence' unique ID in the case of deleted sheets this has no other implications for the tools.