Custom categories limit

Setting up a FortiGate for multiple distinct schools (30). Each requires custom web filters. We manage all sites under one VDOM.

Configuring custom web filter settings were a category is blocked and they want to allow a specific site. Block social media but allow Facebook. Once a category is blocked, an allow does not work. So we started to create custom categories for web rating overrides. We must of hit the limit of custom categories "maximum number of entries has been reached", we have 52. Is there a way to increase that number per VDOM?

v7.4.8