I cross-referenced the Core Book Mounts and "Ammo Bins" on Pg. 159 of Rigger 5.0, and came to the following conclusions sans errata
Vehicle Light Mounts come with a magazine of 250 rounds. For simplicity's sake.
Vehicle Standard Mounts still come with a magazine of 250 rounds.
Vehicle Heavy Mounts still come with a magazine of 500 rounds or BOD rockets/missiles.
Heavy used to count as 2 mounts and had double ammo, so a full reload would take 2 Ammo Bins.
There doesn't seem to be a way to increase your missile count RAW. I'd go with Ammo Bin adds BOD/2 missiles.
Go figure Lasers out yourselves