Assigning a “serial number” to each reserver (need placeholder)

Hi,

I want to assign a unique serial number to the reserver like this:

Event A –
John #1
Bob #2
Mark #3
and so on

Event B
Mary #1
Chris #2
Tiffany #3
and so on

– multiple events simultaneously being reserved so the serial can’t be global
– need number to reset after new event published

Currently, I use the #_BOOKEDSPACES for this – it works in most cases but say if

– 20 seats reserved
– Joe reserves 1 seat > he’s #21
– previous reserver cancels 1 seat (BOOKEDSPACES now at 20 again)
– Billy reserves 1 seat > HE’S #21

Hence there will be an overlap and is where this method fails.

So I need a placeholder for this.

Can anyone propose a way to do this, or know a workaround?