Maximum number of variables in Rules
I have seen some pretty stupidly complex rules with variables and if you remember back to IdentityView, we had some rules with say 5,6 or even 10 variables in there - and this is before we had local and global variables - hence the potential for performance hits if you got IdentityView wrong!
So I have seen 5+ variables and it does work, BUT (and its a big one), the more variables that you use the more the potential extended time to evaluate the rule will occur. While the way that the correlation engine works is to have this evaluated at speed, you run the risk of hitting problems with a rule taking an extended time to complete its evaluation as it branches to the variables.
But the overriding guidance for rules still applies - dont do a complex use case in one rule - while it works, its also not the most efficient and you won't be getting the best evaluation process on the rule itself (elimination evaluation is best and most efficient - for example, using AND evaluations is best). So look to break down the rule, simplify and maybe spread it across several rules rather than just one complex one. Same goes for variables, look to use simpler ones and minimize the impact - break the use case down and make it easier, and reduce the number of variables in the rule for efficiency purposes.