As a rule we try and avoid having too many roles assigned to a single activity as it tends to cloud who is reponsible for doing the work. To work aroudn this, we reconsider the activity and sometimes it can be broken up into smaller activities (sometimes using parallel activities if that makes sense). However, it does happen and we've hit the limit a few times.

One way we've worked around this is to create team/group roles and assign them to everyone in that team. For example we have IT Employee that we use for tasks that anyone in IT might be responsible for completing, we then assign this to everyone in IT. It means that they now have two roles but Promapp will show them all processes that relate and they'll get all the related notifications.

Thanks so much Damian, we will give that a try.

I have raised this issue in user voice to increase this limit, the work around is create more roles which for us is not an ideal solution. Please consider giving it a vote.  https://nintex.uservoice.com/forums/919189-7-nintex-promapp/suggestions/38871628-increase-number-of-roles-able-to-be-assigned-to-an

6 is the limit of role numbers that can be added per activity. 
If you are looking to place more than 6 roles to an activity, it might be best to split out the different activities or create some larger roles that encompass the users from all the separate roles.