Invoice Numbering

Working on a booking system I need to implement invoice numbering.

Invoice numbers must be sequential and with no gaps. i.e. 1, 2, 3 - is ok. 1, 3, 4 - is not ok.

In my present CRUD implementation I do a Lock, query over invoices table, generate new number, save and unlock.

How do you normally go about it? It must be a common scenario in many systems.

I could use a technique suggested in my other thread on scheduling: (!topic/event-store/o_Y2exIxNZM)

i.e. introduce Invoicing aggregate to accept and number the invoice and then use Saga/ProcessManager to issue command to create the actual invoice.

Is this the best I can do or are there simpler solutions here?

Put invoices in a stream, use event id generated in that stream (they
are synchronous)

Shouldn’t I have 1 stream per aggregate?
What if i need to change the sate of the invoice - for example: IsPaid? That’s one more event in the stream.