There are times where maps are not that full, because not many players are playing, which is perfectly normal. Your server has nothing to do with it, because GW2 uses mega servers, where all players from a region are basically on the same map instances. Your server is only relevant for wvw right now.

The game dynamically adds instances of a map, when more players join a map, to keep them at a better performance level. When players leave, instances close again. This is best visible when metas happen

You can have bad luck and get put in the “last” instance with the few remaining players, but that shouldn’t happen that often. If you see are on a meta with few players check the lfg for maps with commanders, they are usually full of players.

The commanders/mentors you see are probably players temporarily activating their tag for a single rift/event and are not doing a longer rift hunt squad.