Go to OV Configuration Workspace and from there select Presentation/Search. Double click "Service Call" to open and there you can change the query restriction to a desired number. Though you can define "No query restriction" a number about 10000 is logical. Bigger numbers will lead to low performance.
Yep, configuring a bigger number will lead to performance issues. From my point of view, 500 itself is logical. No helpdesk engineer will have more than 500 open calls assigned to him. Even if you increase the limit to >1000, it's always better to limit the data being displayed in the views, using filters.