The number of jobs displayed in the timeline widget (BuildTimelineWidget) of the build history page is limited to an .(see https://github.com/jenkinsci/jenkins/blob/master/core/src/main/java/hudson/model/BuildTimelineWidget.java#L49)
It would be a great improvement to make it configurable, through whatever way is appropriate (setting in Jenkins configuration page?).
The timeline widget is a great tool to debug issues that may have happened during the night's builds (job overlap, conflicting ressources access, etc). But if you have more jobs than this limit, its purpose is defeated.
Other people were requesting this in the comments of this old issue: https://issues.jenkins-ci.org/browse/JENKINS-17405