When adding more variables to workflow Catalog Task activity, all the variable names are added in a Slush Bucket field. And this Slush Bucket field has max length defined of 4000 characters.

The "Variables on Task Form" slush bucket is limited to 4000 characters.

1. Try to remove variables that no longer used to make more space for your new variables.
2. Try shorten some long variable names to make more space.