If it's a closed rectangular room, the free REW Room Simulator can give a pretty good estimate of frequency response with different quantities and sub&listener locations. Check the time alignment box. If you plan to use EQ, the main goal is to minimize nulls in the listening area. Without EQ, the goal is for the flattest natural frequency response in the listening area.
Here are a few layouts that I have found work best for minimizing nulls):
1) 4 subs in the corners (most output, good seat to seat consistency)
2) 4 subs on 1/4 3/4 positions on the front and back walls (best natural response, less output, good seat to seat consistency)
3) 2 subs wall centers on opposite sides of the room's longer axis (best natural response, less output, good seat to seat consistency)
4) 2 subs in opposite corners (more output than #3
, can EQ for good sound in 1 set, poor seat to seat consistency)
5) 1 nearfield sub (less predictable, but best way to get good results with 1 sub)
3 subs can be tricky, since you can't place them symmetrically. If you are against the back wall, it might work to have 1 behind you and 2 in front, since you would need less output so close. It's hard to get good frequency response when you sit close to a wall, though.
In general, I've gotten best results with #1
With 4 subs, you also have the option of placing them in pairs for the same results as #3
I got better results with #1
, but YMMV.
If you need room for the center speaker, one good option is to place 2xS1s on either side of the center speaker, and 1xS2 in back, for what is effectively layout #3. You can experiment with this kind of stuff in REW.
You can also place speakers on top of subs, if that gives you the right height.
For the S1s and S2s, overall, I'd suggest one of these, depending on placement practicalities:
1) 4xS1s in corners.
2) 2xS2s on the center walls of the long axis.
You can also do 2xS1 on the center walls of the long axis, although with less output. That might be enough, depending on your requirements.