The number of episodes of the show This is Us is 18 per season, and so far there are three seasons. The first series premiered on September 20, 2016 and it turned out to be so popular that two more series, both with 18 episodes were then booked.
It will become evident, if it hasn't already, whether the popularity has remained high enough for a fourth season to be booked. If so, this will air in 2019.