That service parameter is global, so if you modify it that will affect your redundant 6608 also.
Set this value to whatever maximum number of participants you want in a single conference call. 6608 supports maximum 16 participants, so you may want to set it to 16 to use that functionality.
Also:
*For a G.711 or G.723 conference32 conferencing participants per physical port; maximum conference size of 16 participants
*For a G.729 conference24 conferencing participants per physical port; maximum conference size of 16 participants
So, you can have more than 2 conf sessions, provided the total number of participants donot exceed the stats above, for any given port and codec.