Limit on number of assets customizable in field layout pages

[proimage]

Description

Sometimes an image field may only be set up to handle one file, and sometimes it's used for an entire gallery.

With the flexibility that configurable Title and Instruction fields provide, it would help reduce the number of custom fields required for a site if Asset fields could also have a custom number of selectable files. Something like this:

[brandonkelly]

This idea was already requested by #4321.