1U stands for one unit and it is a standard unit of measurement for rack-mounted servers. It is equal to 1.75 inches (4.45 cm) in height. A 42U rack can accommodate 42 1U servers or a combination of servers with different unit sizes. Therefore, the unit size per server should be 1U if there are 19 1U servers in a 42U rack.
References: https://www.comptia.org/training/resources/exam-objectives/comptia-server-sk0-005-exam- objectives (Objective 1.2)