If you have an Arc (This provides the FR, Centre, and FL), the only option is 1 or 2 Sub and 2 rear surround speakers.  

Thanks for your answer. So what will happen if I connect it? No sound? No rear effect?

You can’t connect a single speaker to act as a rear.  But you say in your first post that you have two Sonos One SL’s as FRONT left and right speakers, these will be outputting your REAR left and right channels.

You mean that even if they are placed front, they play sound that are happening rear? Again, you say that they work by pair but if there is only one, will it make sound? If I follow you, ARC is the only speaker you can have front (not considering SUB). If I just can have rear speakers, how the right and left effects can be played?

As per my first post: The Arc has 11 speakers within the case that will output the front left, centre and front right channels, so this is where your left and right front effects are played. You can then enhance the Arc by adding 2 x rear speakers and/or 1 or 2 Subs.  The App will not allow you to add a single rear surround and as you’ve already added 2 Sonos One SL’s, albeit positioned at the front, the Sonos App will think that you already have connected 2 REAR surround speakers and will only allow you to add another Sub if it or your existing Sub is a gen 3 Sub.

Edit:

I suggest that you move your 2 Sonos One SL speakers, that you have already connected, to the rear and either side of your seating position, preferably at head height.

For the record, Sonos has no idea where each speaker is physically placed, it only is aware of what type of signal is being sent to that speaker. You can place your ‘rear’ surrounds in front if you so desire, but it will both sound unusual, and not provide the ‘surround’ experience intended by the creators of the content, as they are assuming ‘surround’ means behind the viewer’s position. 

If I buy another One SL to pair it, can I have two pairs, one for rear and one for front or sides? I insist because I have no answer: what will happen if a add only 1 One SL in addition to my pair? No sound? If any sound, what it’ll be?

You cannot add any more speakers to your home theatre Sonos setup (“room”).  So the question of what would happen if you did does not arise.

If you have the additional speaker(s) as a separate ‘room’ in your Sonos system you can group it/them to your HT room to play the same content.  It would sync perfectly for music sources but there would be an audio lag for TV sound, creating an echo effect.

The best thing to do, as everybody else has suggested, is to move your rear surrounds to the rear.  And don’t try to add more speakers to this setup.

Here is my living room config. I don’t know how to place the speakers at the rear because I am limited (A, B, C, D, E,F,G). If you have any idea, I would really appreciate. 

In-ceiling speakers may be your solution for surrounds. Or, F&G is the only “rear” location. But since you’ve been listening to (in effect) just the front 3 channels via your Arc, why not leave it at that? 

I agree with this.  If you can’t actually place speakers in the rear, could compromise with ceiling speakers a couple feet behind the sofa.  If you can’t do that, then you are better off without surround sound and just go with Arc + sub.