There's no fixed number. A good mapper can push their map to ranked easily as their "first draft" is already close to rankable and does not requite a lot of mods. An example is one of the
first beatmapping contest winners, ranked within a week of submission.
On the other hand, someone who is working on getting their first mapset ranked will see a lot of room for improvement and refinement in the form of mods. If it helps,
here's my first (and currently only) ranked map's discussion topic.
As you get more and more mods you should (hopefully) see the "quality" of your beatmap improve with everyone's inputs and suggestions. Eventually you'll reach a point where additional mods improve little to nothing - either the modder has nothing to add, or you are able to dismiss (with good justification) the given suggestions. When a few modders find that it's good to go, then it's probably time for a BN check.
I might sound stupid for saying this but just for the record, do finish your diff spread before approaching a BN. BN's job is to check and nominate complete mapsets with no issues, so they might not even look at WIP sets unless they specifically say otherwise. If you just want confirmation that you're doing the right thing so far, I would recommend seeking out an experienced modder that isn't a BN instead.
tl;dr: mod until you (and a few other people) think it's perfect. could be 2, 20, 200. who knows?