Facts and (post today) supposition
ACP stated 8000 of which 2500 reserved for French nationals.
Availability on the pre-registration mechanism was 7600.
As @ 18 Feb, 5820 have pre-registered, of whom (estimate) 1300 French: 1780 remain shown as available.
Assume 1000 of those will be kept for French (and another 200 released on 27 May - for French only).
1030 pre-registered with a 400 last year.
2493 pre-registered with a 600 last year.
2297 pre-registered with a 1000 (and +) last year.
Opportunity for non-FRA riders to pre-register is now 780: surely we can assume that anyone slow at 1100 UTC on 25 March will find pre-registration closed. Suspect pre-registration will remain open for FRA nationals.
27 May: Registration proper opens, but additional starts released only for FRA nationals (up to the balance of 2500)
[Registration requires 3 BRM homologation numbers (select any 3 from 200, 300, 400, 600 (or longer ride(s) in lieu).]
20 Jun: Any pre-registered riders who have not converted their pre-r to registration (and paid full entry fee balance) will have their pre-registration 'cancelled'. Last time there were more than 1500 cancellations and these slots (starts and start wave slots) were 'released' for others to register (as in 'enter').
Anyone taking this route (I am one) will need three BRM homologation numbers (select any 3 from 200, 300, 400, 600 (or longer ride(s) in lieu)) ready to submit as part of that entry.
"You can register before having completed the last qualifying brevet, but your registration will be cancelled and not refunded if we don’t receive its homologation number, at least on July 2, 2023 (at midnight French time)."
2 Jul 2300 UTC-1 = BST: All those who have registered (whether pre-registered or otherwise) must have provided all 4 BRM homologation numbers by then.
In UK (we are lucky) there are nine BRM 600s in the period 17-24 Jun thanks to the great efforts of all the organisers for the last.minute.com merchants and AudaxUK has systems in place to turn the submitted results from those round in short order to allow riders relying on a 'result' to have that last homologation number.
https://www.paris-brest-paris.org/en/how-to-register/