Use "Tools" -> "Select Elements" to select only nodes with in-degree 0, then use "Tools" -> "Analyze Graph" to count the currently selected nodes.
In "Tools" -> "Select Elements", tab "General" disable "Act On Current Selection Only" and "Select Inverse" but enable "Clear Selection First", then disable "Use These Criteria" on all tabs but the "Nodes" tab (where it has to be enabled), finally set "Select" to "Indegree" and "Minimal Degree" as well as "Maximal Degree" to "0" on tab "Nodes".