What size screen can fit 6 clients without stacking?

Yeah you're probably fine using that with an external monitor.

 

I never thought of doing that so simple but brilliant

 

I forgot to mention my resolution is like 1080p

So its that 8gb, 750gb and i7

 

It depends what i7 and what your GPU (if you have one) is, but I'd honestly be surprised if it couldn't handle it. Unless you're planning to use OSBuddy or another client with poor optimisation, osrs isn't a particularly demanding game when on fixed size mode.

Resolution is specific to your monitor and can be changed to whatever you need up to its maximum. Your laptop screen is 1080p but your external monitor could be 360p or 4k (3840x2160), just be reasonable with what your setup can handle.

Macbook pros (late 2013) and after can supposedly 'handle' 4k monitors at 30Hz - Using 4K displays, 5K displays, and Ultra HD TVs with your Mac
You should be able to get away with a 2560x1440 monitor just fine, probably even on a slightly older model.

 

I have a MacBook Pro (13-inch, Mid 2012)
2.9 GHz Intel Core i7

This is good right?

 

I've got this setup.
https://puu.sh/AiiHA/507a73b029.jpg
2 Externals + Laptop Screen
External Monitors is Dell 24"

Total Screen Resolution: 5757x1077

 

Should be fine, make sure you have the right cables since apple usually have their own unique ones.

For reference - Mid 2012 13" MBP 2560x1440 | Mac Forums

 

I can fit 4 on 1980 x 1080