Actually, I think it is more accurate to say that the limit is 15 + 1 (rather than 16). This is because one of the members can be "SelChan," which means that:
If you assign a scan list to Channel X, and
Channel X is not a defined member of the scan list, but
"SelChan" is a defined member of the list, then
Whenever you have scan invoked with the channel selector on Channel X, Channel X is temporarily added to the list.
This is a very flexible feature. It enables a very short scan list to be usable over a broad range of channels. For instance, the user is a radio issued to a piece of fire apparatus. There are 50 fire departments in the district, to any one of which the apparatus might be assigned on mutual aid. The operating channels for all 50 departments are programmed in the radio, and all tagged to the same scan list. That list consists of SelChan (Priority #1), user's home channel, and district metro channel. This three-member scan list works just fine on all 50 of the channels.