That’s a 320 amp service, which is sometimes called a 400 amp service for slang (don’t ask me why, because I don’t know). Really and truly, it’s max rating is 320 amps. An electrician would refer to that service as a “three-twenty”.
No, there is no limit as to how many downstream receptacles may be protected by a GFCI breaker or receptacle. In real life, one would try to keep this to a reasonable amount to assist in later troubleshooting when it trips. I, personally, favor point-of-use GFCI receptacles at each location where GFCI protection is required. This eliminates lots of headaches later on, but at additional initial cost. This exterior circuit you saw, with 15 downstream receptacles on the GFCI, while legal, is just nuts. It’s the outdoor equipment and outdoor receptacles that cause the biggest headaches trying to sort out what tripped the GFCI. As an electrician, I’d highly advise you to note that in your report as a very probable nuisance in later years, and consideration should be given to point-of-use GFCI protection at each exterior receptacle. Many manufacturers have a functional limit of 250 feet of wire on the load side of the GFCI also.