You can have a Validation script on as many of the fields as you want; they’re all available:
Things to be aware of:
•Pre-existing data that does not conform with your new or modified Validation Extension, could be changed by a Save, or cause the Save to fail until the field data is corrected
•Changes made to the data by a 3rd party product, where the changed data no longer conforms with a Validation Extension, could be changed by a subsequent Save, or will cause a Save to fail until the field is corrected