If all else fails you might be able to find an original part number here (
https://www.gcp.se/en/catalogues/model/400-en/ ) but it will take some wading through.
Unfortunately Volvo seemed to lose interest in the 400 series; they're very poorly supported now and finding parts can be difficult.
It's not much help but as far as I recall it's a single polyvee serpentine belt. However the engines were sourced from Renault so you might be able to find an equivalent Renault part
Martin
one time owner of a 1994 440