I configured an ua server and ua client to create multiple sessions and multiple subscriptions on the referenced ua server.
with this scenario I created 800 sessions, the sessions creation succeeded as shown in the "sessions creation succeeded" figure.

After the sessions creation succeeded, with my client i created a subscription for each session , so in total 800 subscription should be created, but in the process of subscriptions creation i witnessed that after a certain number of subscription creation my server started to close the previously created sessions and the number of total created sessions decreased from 800 to 504 as shown in the "closing sesssions" figure.

Then after some time all sessions were closed and the created subscriptions persisted and were not deleted as shown in the "all sessions are closed" figure.

Finally subscription deletion took some , and the created subscriptions were all deleted as shown in the "Subscription deleted" figure.