You can define individual capacities of project members.
Open Projects > Timeline > ⚙ icon > Capacities
In the following dialog you can set the working hours per person per week. Alternatively, you can apply the default capacity to all by checking the Apply default maximum capacity to all checkbox.
Note: The default setting is 40 hours per person per week.
The capacities are displayed in the Timeline > Workload with a dotted line when the mouse is moved over the workloads. In addition, the respective workload is displayed in hours: