A sitting is basically any number of exams you take in any given exam week, so if you book and take one exam that is one sitting used up if you were mad and had studied very hard you could book and sit all 14 exams in the same exam week so 14 exams one sitting.
Your approved course provder will normally have the subjects broken down into 2, 3 or 4 sections or modules as they like to call them, you will study a few subjects then take those exams when you are ready, that is one sitting.
I did mine in three sittings.
1, Met, P.O.F., M&B
2,GNav, RNav, Instruments, Performance
3, Flight planning, HPL, Ops, Air law, AGK, Vfr & Ifr comms!
Don't look to far ahead it may make you unwell!
Good luck.....