This will depend entirely on what they are doing at the time. If they are just holding onto it with one hand, you can fit more than if they are doing leg stretches. Typically a 4' barre is recommended for use by just one person. For barre lengths over 4', generally allow 2' per person. The barres may be used on both sides.

A double barre is beneficial when there are multiple users of different heights. (ie. a teacher and a student) Even for a single user, having the second barre is a luxury so that you can have both barres set at heights you desire without needing to change them.

For some barre fitness programs a ballet barre will work fine, particularly if you are using the barre to replace using the back of a chair. The ballet barre is intended to be used to keep one's balance and is not designed to bear full body weight, so if you are using it just to keep your balance it will work fine. Other barre fitness programs are more strenuous and require pushing and pulling and weight bearing which a freestanding ballet barre cannot withstand. For those programs, we recommend a wall or floor mounted barre or a FIT Barre™ freestanding Fitness Barre.

They do not "fold". For the Prodigy Series, you may loosen the knobs on the feet and pivot the feet sideways to store the barre flat, under a bed or leaned against a wall.

Yes, they are easy to assemble and disassemble as required. The Prodigy Series barres have clamps with hand knobs/handles, and no tools are required for assembly.