Just looks a bit bare on some branches only have a couple of leaves at the end of a branch

Uh, whole tree pic?

My general rule of pruning lemons (planted in ground)... first I prune off any limbs growing in a downward direction, any limbs crossing the center; and any fruit that touches the ground; that is not so important for container plants; but fruit that touches the ground in the field (either from too many fruit or too weak limb) invites fungus, rats, physical damage, etc., meaning the fruit/limb will never be commercially acceptable.

I wouldn't prune anything unless it turns brown. Nice mini tree

Don't prune that. Keep up your routine it looks good and happy.

Ok, I was just concerned that some branches had lost most of their leaves and now just have two leaves at the end.

I wondered if I cut it back wether a new branch would start growing.

I think the reason for the leaf drop may be that it's indoors instead of the greenhouse it was raised in. A lot less light. It looks good right now.

What everyone said.

And as long as the bare branches are still green they'll put out new leaves and maybe more branches in a while. Maybe a few weeks, maybe a few months. Citrus grow in spurts, not continuously. Most varieties have between two and four growth periods a year. Just be patient.