Hi and welcome!
There are a couple of things that might help. Take a look further along in the pattern and see what it says for finishing or for the neckband or collar. Those directions may tell you what to do with the center back sts.
If you start working the right shoulder and finish with 52sts, then as GG says, you're should be on target to bind off the entire 52 (17sts twice and 18sts once). The center 39sts are likely part of the neck finishing.
Can you give us a link to the pattern or a pattern name?