You can add shelves in any supported configuration - there is not need to match or balance shelves between controllers. You might want to for various reasons, such as to balance worload or such between controllers, but there is no requirement to do so. If 47 spindles is what you need on one controller is what you need, either for total storage available in an aggregate or for IOPs then that is what you need.
Follow-up question on your planned aggregate raid-group layout: what size disk? If these are two shelves of high capacity disk, you may want to consider two spares rather than just 1, simply to account for the time it takes to rebuild a single disk within a raid group as the capacity goes up. You have a (relatively) smaller controller and matched with a high capacity disk and sufficient total workload can combine to take a while to rebuild a single disk. I just like to have another spare ready to go (personal preference). If you have a quick replacement time guarantee (like 2 or 4 hour part delivery) and high performance disk, I'd be less concerned. Of course you may also have additional spares of the same capacity from the first two shelves on the controller, which counts as well.
The second point on disk size is that if these are performance disks, you coulud go with a higher raid size and reduce the number of raid groups, thereby getting a little more capacity. My personal preference is to use larger raid groups and reduce the total number of parity disks within an aggregate - for instance a RG size of 23 with two spares makes up for the extra spare - 42 data + 4 parity + 2 spare across two raid groups as opposed to RG16/15 - 41 data + 6 parity + 1 spare. If you are using high capacity disks the max raid group size is 20 so it's not as easy to match up with 48 disks total.
Hope this helps.