I believe the seat map isn't 100% accurate if everyone hasn't checked in for the flight. At least that's what I've been told. I use a technique of pretending to book whatever flight I'm trying to catch, on the united app. I start with 9 adults (most you can select), and go down from there, until I arrive at the number of adult seats it will let me purchase. This should indicate how many available seats there are, and has served me well! Of course if you're really nervous, people do seem to post responses to load checks fairly quickly on APC. Good Luck.