All Apocalypse World rolls are 2d6 + "bonus", with 6 or less counting as a "miss", 7 to 9 counting as "soft hit" and 10 or above counting as "hard hit". Most rolls decide the bonus part by a stat, some use expendable resource (like Angel kit's supply or Barter).
Stats are capped off at +3. Other rolls that allow spending a resource for the bonus (like using the Angel kit's supply, or "dropping jingle to speed an item on its way to you") specify the maximum bonus at +3 too. This has made some of my players assume +3 is the maximum bonus one can get, and they ask me about it. I don't really know, myself.
With +ongoing or +forward, they could get +1 to their roll, and successful help from another character is also +1 (+2 in the previews of Second edition). This would make the total for a helped character with +3 bonus and +forward/+ongoing as high as +5 or +6. Both outcomes are pretty good; +5 would never miss, and be a hard hit 5/6 of the time. +6 never misses either and gets a hard hit 11/12 of times.
I'm not questioning the power of these modifiers; as the GM it's not my job to deny the players bonuses, and I'm pretty sure any character who's got +3 bonus, help and +ongoing has already earned that hard hit. I'm just curious - is there a rule, somewhere, that puts a limit on bonus modifiers placed on a single roll?