Hi!
I think 5 is reasonable. In many countries postage rates are high, just for exchanging 2 sheets we have to spend more for postage which is not worth. This will reduce the number of participants. I will think twice if it is reduced to 2. The Souvenir sheets need not be from one's own country. So once you received a return SSs, you can keep your themed SS and rest you can sent to the next participant.
To increase the participants, my suggestions are,
5 MINT souvenir sheets (MINT should be forced). Cindrellas and CTOs cann't be displayed in stamp exhibits and it is of no use to the real stamp collectors.
Atleast 2 of them from the recent issue, issued after 2010/2015. (most of us sending single stamp SSs that too from old era which is very much available in flee market)
Atleast 3 Souvenir sheets should have 2 or more stamps.
And on the cover commemorative stamps should be used.
The number of exchanges per person should be increased.
Thanks
iphil