I believe certain older systems & releases *may* have supported up to 52 disks but I'm not 100% certain.
Luke is correct. Once upon a time, Data ONTAP only supported one volume (file system), underpinned by one RAID group. The largest configuration that was ever made available during this period was the 468GB (raw) F630, which achieved that capacity using 52x9GB drives.
When multi-volume, multi-RAID (MV/MR) functionality was first introduced in Data ONTAP 5.0, it was decided to reduce the maximum RAID group size down to 28. This was not because 52 drive RAID group configurations had ever shown themselves to be a problem in practice (they hadn't), but because it just seemed prudent given the new capabilities. As I believe most of the folk on this list will attest to, the default of 14 drive RAID groups yields tremendous reliability, at a parity-cost that most customers are happy to accept.
Anyway, when MV/MR came along with its then-new 28 drive maximum RAID group size, the software continued to support the existing/preceeding SV/SR configurations that involved more than 28 drives, such that those configurations didn't have to be disrupted or reconfigured to run the new code. This was, and still is, handled as an exception to the "new" MV/MR rules.
Keith