I think it doesn't matter what jersey number he uses, the most important thing is that he also becomes a reguler player for Arsenal because it would be useless if Arsenal bought him at a fairly expensive price but only used as a reserve player, for a 28 year old player I think 39 million euros is quite expensive.
Merino market value is 55 Million Euros, so technically Arsenal bought him at discount.
This is a good signing by Arteta, currently there are no good midfielder in Arsenal, Odegaard is supposed to be a player who can create chances to the strikers, but from the last season he's under perform.
It's always Saka that carry Arsenal, but this season Havertz is back and then Merino can support both Saka and Havertz to score more goals.