As a general rue, a luggage is deemed as an item that can be carried all at once. 

For example, these would all count as 1 luggage: 

A bag connected to top of a suitcase
Multiple Items of the same type bundled into 1
Ski Boards, Golf clubs in a golf bag

However for locations that have local luggage rules including Tokyo Station luggage storage, local rule will have priority. In this case, all luggage may be counted individually regardless of size or type.