Per FAQ: There is a limit on which relatives can be included in a tree, based on their distance from the tree's founder. This limit helps ensure privacy for family members, and prevents the tree from growing indefinitely. If you reach the limit, click the 'Create new family' button to start a new family branch from the selected person.
The forum seems to have various explanations. If I am the founder, what is my limit on adding blood relatives and non-blood relatives?