Considering this, are Korean dramas only one season?
Most Korean TV shows are called dramas. Most run one season-13-30 episodes. 16-24 episodes is the norm. There are also longer dramas and sitcoms that run 50-100 episodes often called dailies (think soap operas).
One may also ask, why are Kdramas all 16 episodes? Most of the series have allotted episodes based on the story structure. 16 episodes are ideal numbers as it's not too long or too short, so it turns easy to fit the story structure in it without making the series dragged.
In respect to this, do K dramas ever have second seasons?
It's common for K-dramas to only go up to a certain number of episodes. This means many shows only make it to one season and it's unlikely a second installment is created. K-drama fans do have a glimmer of hope as some shows broke the mold and went into a second season.
How long do Korean dramas last?
Series set in contemporary times typically run for a single season and usually contain 12 to 24 episodes of 60 minutes each. Historical series may be longer, with up to 200 episodes, but they also generally run for only one season.