XOP Gate --> Is there a limit to the number of parallel instances coming to t

Yes, that's the expected behavior, the OR-XOR gateway waits for the other incoming flow to be activated. That's why this is not a best practice and instead you need to use a script task before (even if it's an empty script task) to join all the incoming flows and direct a single one to OR-XOR.

This is explained in our documentation forum.appian.com/.../OR_Node:

"When multiple flows enter an OR node, the Gateway node pauses after the first instance token passes through – until all other incoming flows arrive. Work around this issue by placing an empty Script Task node between the incoming flows and the Gateway node. "

I do have a Script Task in between... But I execute more than 10,000 instances parallely and then at some point it stops at one of the calls....

The problem is that your AND has two incoming flows: "Delete Reject Cancel Report Document"and "Check RET Time" but only one gets activated at a time but the AND requires all flows to get activated to continue with the execution.

The solution is to add an empty script task to concentrate both incoming flows. Please take a look at these screenshots for a graphical explanation based on your model and on a test model I created:

forum.appian.com/.../85966
forum.appian.com/.../85967