A wheelchair is a chair that a person must use when they have a hard time walking, or cannot stand up. It would have 4 wheels, but a wheelchair is singular and only one person can sit in it. A business, store, hospital, etc. may have a few wheelchairs they can let people borrow. That means there is more than one wheelchair, so more than one is plural, (wheelchairs). If there is more than one chair / wheelchair, they are plural. If there is one chair / wheelchair, it is singular. The amount of wheels does not matter. We're counting chairs.

Do NOT focus on the number of wheels. By the way, wheelchairs almost always have 4 wheels. The chair is what you focus on. You use wheelchairs only when you have more than one CHAIR. "I have a wheelchair reserved, but I need a total of three wheelchairs."

Wheelchairs is the plural(more than one wheelchair) form of wheelchair, it has nothing to do with how many wheels it has. However if you talk about a specific wheelchair's attribute for example, You can add an S, "That wheelchair's seat is nice" but then you need to remember to add an apostrophe before the S. Hope that made it more clear for you.

1 wheelchair 2 wheelchairs it's just plural.