Well, I found a solution with 2 Portals.
One with 3.
The easier version of that solution with 4.
And another solution with 4.

This map is definitely easier than it looks, which is why players are not gonna like it.
Make maps that are harder than they look to challenge players.
Make maps that incorporate every puzzle element into the solution.
Make maps that don't have THAT many solutions.

Also, take a look here: Building great maps

Hmmm... Well if you try to reach the exit the ninja way maybe only 4 X)

Actually, only 4 are needed. Like I said, easier than it looks.

Well, I don't know if I did it the right way, but I did it... About 8 portals needed to complete it if I remember correctly :p