In general, more disks means more parallelism, more IOPS. However, this is not applicable to 'Optimised Specialised Storage Systems'. These days most of the newer NetApp systems do 'abstracted IOPS' (Running :cDOT/ONTAP). The total IOPS that a node can provide is based on the physical characteristics of the node—for example, the number of CPUs, the CPU speed, and the amount of RAM. The total IOPS that an aggregate can provide is based on the physical properties of the disks—for example, a SATA, SAS, or SSD disk. Hence, the combination, CPU cores, RAM in your Node, and the number of volumes on that 'flash-pool, IOPS availability will differ but will be way greater than the actual RAW IOPS. It's bit complex but I guess its projected based on a generic workload based testing and it's different for different filer Model.
Regarding your specific query: More disks you have in your raid-group set, more IOPS will be available at RAW aggregate level (That doesn't mean putting all in the single-raid_group, there is always trade-off). As I mentioned, the actual front-end IOPS will depend on many different factors.
Considerations for sizing RAID groups:
https://docs.netapp.com/ontap-9/index.jsp?topic=%2Fcom.netapp.doc.dot-cm-psmg%2FGUID-EE30CDA9-095F-4DD1-9A29-BB9F0E55DB4F.html&anchor=:~:text=When%20y....