Numbers don't really mean much these days in relevance to player's role in the team. But he'll surely get a first team number (between 1-25). Could be before the Elche game although with no fans in the stands I don't think there will be a traditional presentation of the team like usualy at Gamper this year.
Another problem is we need to free up some numbers before we can register Barca B players as first team members.
Here are currently (officially) unoccupied numbers:
4 - Araujo?
6 - Alena?
12 - free - last worn by Rafinha IIRC, so if he'll stay at the end could get his number back but I think he'll leave
17 - Trincao?
And then we have players who could potentially leave:
2 - Semedo - this number would probably go to whoever would replace him if he leaves
9 - Suarez - Depay would probably get this number if he comes
16 - Wague - he's surely leaving, at least on loan late in the transfer window if we won't find a buyer
19 - Braithwaite
22 - Vidal
23 - Umtiti - sadly don't think we'll manage to offload him
24 - Firpo
And then we need shirt numbers for following players:
- Pedri - wearing #27 in preseason
- Puig (28)
- Fati (31)
- Todibo? (if he stays which I dont' believe)
assuming Araujo, Alena and Trincao will stay with the numbers they're having in preseason.
Pedri (unless his contract prevents that) and Puig could eventually still be registered as Barca B players (so wearing their current numbers) but Ansu will need to be given a first team number to extend his contract.