score:4
Like in many other cities the normal taxi is a car for 5 people (a driver and 4 passengers.)
It's possible that some taxi drivers haven't a problem with transporting an extra passenger, when this is a young child.
In your case I recommend to you order a special taxi ahead of time by phone.
Here are a list of some taxi companies:
In my personal experience with taxi drivers in Lisbon (and I'm a Portuguese speaker) it is quite common for the driver to take advantage of the passenger.