How many passangers in a taxi ?

We are 4 travellers who want to travel in Tokyo area. How many passangers are allowed to sit in a standard taxi ?

The standard taxi fits 4 passengers no problem, three in back and one in front. More info on this page:

http://www.japan-guide.com/e/e2021.html

We have caught taxis with 5 of us: but not every taxi can do this...you need to indicate at the taxi stand how many you are and if in luck a 5 seater will be around.
The 5 seaters we have used have 3 in the back, 1 in the normal front seat, and one in a narrow sort of fold down-seat in the middle of the front which makes the front like a bench seat. BUT it needs to be a very skinny person in that middle seat!!
As I said, not EVERY taxi has this configuration.

Yes, those are large size taxis (oogata taxi). They're not as common as the medium and small size taxis. More info at that link I posted.