Like Voyager or Voidwalker, maybe they could add an Entrepreneur rank or CEO rank? Entrepreneur would give you 15. CEO Would give you unlimited.
By the linear progression I exemplified, EcoLegends would already have 7 slots available, which I think is pretty enough XD I'm assuming that, when you post a job and some players apply for it, you can accept more than one for the same job. Under this assumption, I think I can safely estimate that >95%¹ of people that will ever use this system in their entire life will never need more than 2 slots. There's another consideration: if you give someone more than they had in the first place, they're gonna be happy; if you give someone less than they had in the first place, they're gonna be angry; this, regardless of whether they ended up with the same amount in both cases. Translation: the amount of slots per Rank can always be increased making everyone happy; while on the contrary, if we start with too high numbers and are later forced to lower them (for spam or any other reason), people are going to complain. And we don't want /job to become too crowded, because people would be going to ignore it, just how they currently do with trade channel. ¹yes, I took that number out of my ass, it's just to give a rough idea for the sake of clarity
This was fun to read, I think that we should give them less to start with as you said. This avoids spam and also makes it so we can add more later if needed.