What exactly is a watch winder?

A watch winder is a device used to keep automatic/self winding watches wound when they are not being worn. Automatic watches operate on the principle of winding themselves using a moving weight inside the watch.  The weight swings or rotates while the watch is worn and turns the winding mechanism inside the watch. So, fairly obviously, if the watch is not worn, then it no longer receives power this way and will run down. While virtually all automatic watches can be manually wound, this is not always convenient. So the concept of an automatic watch winder was born.

A watch winder is a device which holds a watch (or often more than one) and moves it in a circular patterns to emulate the necessary parts of human motion to operate the self-winding mechanism.

A winder cannot over wind an automatic watch, since all automatic watches are protected from being over wound by a mechanism that disengages the winding process when the mainspring is fully wound. But using a timer-based winder is still very important to prevent excessive wear on the winding mechanism. There is no need to keep the watch in motion 24 hours a day when usually only 30 minutes of motion is necessary to keep it properly wound.

Winders are totally useless with battery-operated quartz watches. But they do work for the special 'battery-less' quartz watches, such as the Omega-Matic, Seiko Kinetic, and similar. These automatic/quartz hybrids use the same weight/rotor principal to generate electric power to run a quartz movement.

Do I need a winder or not?

For automatic watches you wear frequently a winder is a great solution for keeping them ready to wear on a moments notice.

The single biggest advantage of a winder for seldom used watches is for those with complex perpetual calendar features that may require elaborate resetting after the watch has been stopped for a while.

Should I start with a single head unit or go for a multi headed unit?

If you are enough of a collector to want a winder, don't short-change yourself by skimping on a winder that can only handle one watch. If you've got the watch 'bug,' it won't be long before you buy another automatic and be caught short. Consider at least one of the single-unit watch winders have optional two-watch heads. Or buy a single-watch unit for starters and plan on adding a multi-watch unit later.

What else should I consider?

Don't underestimate the value of manual wind watches for your collection. They offer the joy of needing to interact with your collection every day or two to keep them going. It can be nice to be needed. Vintage manual wind watches are readily available, modern ones are less common but still made. Not only will you not need a winder, they are intentionally easy to wind by hand. Many automatics have screw down crowns or are otherwise not as accommodating for frequent manual winding.

Automatic watches are intended for fairly regular wear. So concentrate your ownership of automatic watches on those that you will wear at least a couple of times a week. Winders are a nice convenience for watches you don't wear daily. But why spin away most of the lifetime of several nice watches waiting for you to seldom wear them.

A bad watch winder is worse than no winder. Automatic watches are designed to be worn by people. People move in a large diversity of manners, causing a fairly even distribution of motion of the watch's internal winding mechanism. A watch winder is normally limited to a very finite number of directions and types of movement -- some as little as one single motion. So a poorly designed winder (especially the one you tried to make from an old electric drill and spare auto parts when you first heard that a real winder sells for hundreds of dollars) might cause undesirable wear of the watch's mechanisms from excessive and repetitive movement.