The government's White Paper says there is "no single figure" for this, but that there are believed to be 12,000 EU regulations (one type of EU law) in force, while Parliament has passed 7,900 statutory instruments implementing EU legislation and 186 acts which incorporate a degree of EU influence.

errata: 7,900 statutes + 186 statutes. My bad.

So.

How eager would the EP, EC, and Council need to be in order to accommodate an orderly UK "transition"?

Diversity is the key to economic and political evolution.