6 Lavatories in one independent house
Assumed that you operate hotel business in Sumida ward, Tokyo, Japan.
You own 3 stories building and there are 4 rooms in each floor.
Each room does not equip with toilet.
In this case, you need to prepare at least 2 lavatories in each floor.
So, you will prepare 6 toilets in total.
Because, when Mr. M and Ms.F are staying in 3rd floor. They do not want to use same lavatory in 3rd floor.
One lavatory is sufficient
On the other hand….
Assumed that you operate hotel business in Shinjuku ward, Tokyo, Japan.
You own 3 stories house.
When you accommodate guests, a group (they may be family or friend) uses whole house.
They do not share the house with other customer.
In this case, as far as there is a toilet in one floor (ex. 1st floor), it is sufficient.
You do not need to prepare a toilet in 2nd nor 3rd floor.
(it differs depending on capacity of your prospective hotel)