How many threads do I need to earn or upgrade a trade?

Sorry, I misspoke! It is 20 to master. I'll edit my reply above so as to avoid confusion. But the 35 total is correct:

PHASE I: ACQUIRE A TRADE
The character demonstrates the trade generally, including dabbling in one or more of the possible specialties.
Requirement: 5 threads.

PHASE II: SELECT A SPECIALTY
The character focuses on and demonstrates a special vocation within the trade, then spends time further developing that specific skill set.
Requirement: 10 threads (total: 15).

PHASE III: ACHIEVE A MASTERY
The character continues developing their specialty until they are a master of their craft.
Requirement: 20 threads (total: 35).

And I get'cha now! That seems correct (lol, the math has scrambled my brains). You can definitely double-dip Phase I and cut down on your thread load slightly that way. In fact, If you decide to master three specialties for one trade, that means you only have to do 5 + 30 + 60 = 95 threads instead. And since we only allow up to two trades, I suppose the total would max out at 100, not 105.*

* DID I MENTION I AM BAD AT MATH? PLEASE TAKE MY ATTEMPTS TO CRUNCH NUMBERS WITH A GRAIN OF SALT. THANK YOU.