If you attempt to add more than 100 options on a custom field, the error "You can not link more than 100 options to a field." comes out.
When adding options to custom fields, there is a global limit of 100 on the number of options that can be added on a single Kayako instance.
This includes cascading select type of fields categories - e.g., Main Category/Sub Category 1/Subcategory 2 is counted as 3 custom additions.
See the sample below to get a clear picture.
To prevent the error from appearing, check the options you want to add to the custom field and limit them to 100 only.
At this time, there's no way for us to increase the limit for adding options on custom fields. Our engineering and product teams are aware of this issue and this has been listed already on the product roadmap for possible future enhancements.
Limiting the options you are adding to 100 and below to a custom field will prevent the error from occurring.