Use the below query in FetchXML Counter plugin in XrmToolbox to count the workflow sessions in waiting state.
<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="false">
<entity name="asyncoperation">
<attribute name="asyncoperationid"/>
<attribute name="name"/>
<attribute name="createdon"/>
<attribute name="statuscode"/>
<attribute name="statecode"/>
<order attribute="createdon" descending="true"/>
<filter type="and">
<condition attribute="operationtype" operator="eq" value="10"/> <!-- 10 corresponds to workflow -->
<condition attribute="statecode" operator="eq" value="3"/> <!-- 3 corresponds to waiting state -->
<condition attribute="name" operator="like" value="Modified date Ageing reminder"/> <!-- Replace with the name of your workflow -->
</filter>
</entity>
</fetch>