Not very clear in this photo, but I've let all the buds grow, whether on the trunk, or branches.
I prune them off fairly short, and they grow a lot of fruiting spurs.
I also find that many of the buds are spur/fruiting buds and won't grow into branches anyway.
An espalier has very little
wood compared to a 'normal' tree, and I try to leave as much fruiting wood on as I can!
*edit* a couple of unsolicited comments on the pot...
Is the tree on very dwarfing rootstock? If not, I imagine you'll need to repot.
I'd also be careful about what looks like a metal pot?
It could get
really hot in there, and while there's plants that enjoy hot feet, apples aint one of them.