Can a pool's circulation be adequate with only one return jet?

Like most things in life, "it depends"! Yes, one return jet can be sufficient for an above ground pool or a smaller in-ground pool, maybe up to 15,000 gallons or so. But most large in-ground pools have multiple return lines to reduce the size of the returns and improve the circulation.

If your pool has moveable return fittings that let you direct the return stream, it is most common to point them down towards the bottom to improve bottom to top circulation (since the skimmer takes water from the top, its best if you return water towards the bottom).