I know it has come up here a bunch of times before (so sorry for the repetition!); extensive searching has revealed many times this has been discussed, but no conclusive answer (in particular I have not found John Harper himself answering either way).
Is the number of veteran abilities meant to be capped at 3 for characters and 2 for crews (except assassins, for some reason?), unlimited, or is it intended that the rules don’t say either way (mainly because the book does not explain what veteran abilities are at all) and every group should decide for itself?
The last one.
Pretty much everything that is open ended, was intentionally left open ended so that each group could personalize that specific outcome. Partially because of things like players who want to be like an alchemist-tinkerer-whisper mix who can male big booms with nitroglycerin clockwork bombs that can kill demons, or some other really random crap. Could be a basis for a decent Frankenstein character too.
But yeah, it was intentionally left open.
John Harper explained in his AMA that in the core Blades game the number of ticks on the character sheet was a compromise. it was felt that if there was 1 or 2 ticks that a player would not feel free to just pick any veteran advances or worry about it. If you want it from the horses mouth you can see him on his twitch VoD here twitch.tv – Blades in the Dark Dev AMA w/ Sean Nittner at time 03:44:10
Interesting, thanks for the answers! John Potter in particular. The stream (with precise time) clears things up a lot.