Three rings are common but other numbers of rings are also possible. For small projects you may choose to use a simple hole punch to do your own binding. For extremely large quantities it may be more efficient to have the printer or copy shop drill the holes or print to pre-punched paper.
Ring binding is a good option for some types of manuals where page revisions may need to be inserted periodically. Three ring binders come in many sizes -- both page size and thickness.
