How many speed lights can Nikon D610 fire up to?

Can someone tell me how many speed lights can Nikon D610 remote trigger? I own a SB 910, currently I use D610's popup flash as remote trigger. I want to purchase 2 more SB 910, is it possible? Any extra add ons?

Your professional advice is greatly appreciated.

Have you read the instruction manual that can with your SB 910?

Nikon's Creative Lighting System is capable of using many different compatible Nikon SB flash units. I do not think that there would be a limit to how many flash units that could be controlled as long as they are within the range of the system

There are no limits. If you are using optical triggering from the camera's on-board flash as long as all the receiving units can 'see' the signal, there's no problem.

Again, if you decide to use 1 of the SB910's as a commander flash on the camera (to remotely control the power output of the slave units) there's no limit to the number of flashes you can have in each group, but if you want your flashes controlled remotely at variable power settings, you are limited to 3 groups (A, B & C).

As many as you desire as long as you do it within reasonable limits.

I'm pretty sure there's a limit somewhere especially with wired connections. With wireless however, it will depend on how you set it up. Optical triggers need a line-of-sight so as long as the receiver can see the primary flash then it will work. It's almost the same with radio transmitters. As long as the receiver can receive the signal from the trigger then it's okay. How to manage a large number of light sources will be a huge headache though. I'm guessing no shadows at all resulting to a very flat looking picture.

Any number at all, if triggered by simple "slave" rather than by radio trigger.

As many as you need…