I have a custom object with many fields, I need to ensure some of these fields are required during update (but not insert). While I can create a validation rule for each of these required fields, I was wondering if I can combine them. The challenge when I combine them is that I cannot custom my message to identify which specific fields are missing. Is there a way to craft a single rule so that I can apply the violating value in the error message? Also, are there any performance or governance concerns if I create one validation per field. There are around 20 fields to check. Thanks.