Amaryllises do vary in the amount of stems they produce. As you may know, more stems means more reproductive potential. We have had some with three stems!
However, having just one stem can be a good thing because the bulb may have more energy left to grow another stem and new blooms.
When your flowers have bloomed, cut the stem off near the bulb and let nature do its work. There is a pretty good chance there's more to come.
An Amaryllis can live up to 75 years if cared for properly.