Since jinx can stack for every possible buff (up to… duh… 5? So 75% more damage) or for no buff at all (so 0 damage buff), it’s a little hard to compare them.
Since the description state “15% per each buff” we can assume that it does pile up for each buff, reaching some really high picks.
Nope, there’s nothing anywhere about a jinx cap for damage, i just assumed that it is really hard to get more then 5 buffs active at the same time for a single hero.
And even if it is possible, i assume people do not want to make a team that can be countered like that