Every single one. The company we used were only in business to provide this service.
I've remembered the name of the company we used, it was
Parkforce.
It was really easy, pay an upfront fee for the signage and then no ongoing costs. The company insist you place the signs in prominent places around the car park (so one is visible from a distance wherever you park) and then one on EVERY exit, including footpaths. They are large with white lettering on a red background, trust me, you cannot miss them (they have an example on their site).
We then got whoever was on the front desk to keep an eye out for people parking and walking off, we would always put a courtesy note on their car first, record their registration number and then only if they parked there again put a fine notice on their car and take a photo.
Once you sent off the paperwork you didn't have to do a thing, the company did it all for you, we would just get our "slice" (for want of a better word) in the post when the offender paid up.