Pretty close. And you're right about there being a small period when there are only 2 Jesses on board: after Jess 4 is killed by Jess 2 and before Sally 2 dies.
Jess 4? Yep. There are 4 of them. Although, it could certainly be argued that there are only 2 Jesses (Our Jess and Mean Jess), or even just 1 Jess, since they all arrive as the same Jess (Jess 0?).
The 4 Jesses are:
Jess 1 aka "Our Jess", up until another Jess 1 arrives, when "Our Jess" then becomes Jess 3.
Jess 2 - This is the one who arrives next and gets the shotgun pointed at her by Jess 1.
Jess 3: "Our Jess" after another Jess 1 arrives.
Jess 4: "Mean Jess". This is Jess 2 after another Jess 2 arrives.
There are 2 different Jesses going through the loops, and each Jess overlaps an earlier version of herself, as well as the other Jess,
Starting with our Jess arriving on the Aeolus (Jess 1):
Loop 1: Jess 1 arrives. On board: Jess 2 and Jess 3. Jess 1 shoves Jess 3 overboard, go to Loop 2.
Loop 2: Jess 2 arrives. On board: Jess 1 and Jess 4. Sally 2 dies, go to Loop 1.