21 if you count the posthumously released "21".
I have read the series (1-20) twice and it was not an all consuming endeavor with late night page turning. The writing is such that you must spend some time with it in an effortful process. In fact I stopped from time to time to read other stuff. the writing is just so good that you will be pretty disappointed with almost anything else.
A couple of my favorite recollections as best as I can recall:
1.Dr. Matrurin had taken a sloth on board as kind of naturalist observation subject. Aubrey was dismayed that the creature took an almost instant dislike to him while being docile with everyone else on board including the shrewish Killick, his cabin steward. One day Aubrey discovered the sloth approaching him as he was writing at his desk and was able to ply the the creature with a small sip of rum. After taking the drink the sloth retreated and Aubrey went back to work only to be interrupted minutes later with the sloth gently pulling on his uniform sleeve for another drink. Aubrey being pleased with himself continued the practice until one day Stephen discovered the sloth had gotten into some rations and was completely drunk. Irate, Matrurin berated Jack telling him that he had "debauched his sloth".
2. Jack and Stephen were debating the merits of the punishment of death for a sailor who had been caught having relations with goat. The argument was quite heated with Stephen arguing for leniency and Jack for the rule of law in the navy. At one point they were interrupted by Killick serving tea. Jack asked Stephen if he wanted milk in his tea, to which Stephen asked, if it was goats milk...."the" goat's milk. Jack responded, "yes, I believe it is". Stephen relied "perhaps not".
Can't wait.