This plugin shows a Views TabBar like the original one but it adds how many jobs has each view in every tab title.
Go to Manage Jenkins → Configure System. Then, in the Views Tab Bar item, select CountJobs Views TabBar.
The DropDown ViewsTabBar Plugin already implements an option to show the number of jobs in each view. This one, though, sticks to the Jenkins' classic look and feel.
More recent jenkins-core version
Doc migrated to github
Build on ci.jenkins.io