It's not user changeable as such. It's a number assigned by Sonar to indicate the group number it belongs to and AFAIK are assigned sequentially as clips are grouped.
To ungroup select one of the clips, right click and select "remove selected clips from group" which effectively ends the group as all clips in it will be selected.
Create them by selecting all the clips you want in the group and selecting "create selection group from selected clips". A clip can only belong to one group at once though.
For example if you have 3 clips in group 1 and two in group 2, select them and then "Create group from...." you will create group 3 but groups 1 & 2 no longer exist.
Hope that makes sense.