Rembrandt and Saskia van Uylenburgh had four children.
Their children were named Rumbartus, Cornelia, another Cornelia, and Titus. The first three died in infancy, while Titus survived into adulthood. This family history is one reason biographies sometimes mention that only one of the couple’s four children reached maturity.
Rembrandt married Saskia in 1634, and she appears in several of his portraits and history paintings. Their marriage took place during the period when Rembrandt was becoming a leading portrait painter in Amsterdam. Saskia died in 1642, the same year Rembrandt completed The Night Watch.
The number four can be confused with the number of surviving children, which was one. It can also be confused with the number of children Rembrandt had in total, because he later had a daughter, Cornelia, with Hendrickje Stoffels. The question concerns only Rembrandt and Saskia’s children together.