Thank you for your post.
PRTG does not have a native Sensor to monitor the number of (open) tickets in PRTG. However, it's possible to receive this data via the PRTG API. In your case, you can use the "PRTG HTTP API: XML Table Query Builder", which can be found here: "Setup > PRTG API > Live Data". With this feature it's simple to create the required API Call. For example, the following API call will list all tickets with the status "1" (="open"):
<Your PRTG Web Server>/api/table.json?content=tickets&output=json&columns=objid,datetime,priority,parentid,message,user,status,name,type&filter_status=1&username=YourUser&passhash=hash
(Please note that you must enter the corresponding username & passhash)
The received JSON result shows the value "treesize"corresponds to the total number of open tickets.
I recommend to use the API Call in combination with the "REST Custom Sensor" (https://www.paessler.com/manuals/prtg/rest_custom_sensor). Please note that you must create a new template for the option "REST Configuration" in the Sensor settings. This template is used to map returned JSON or XML to sensor values.
Paessler Support Team