Every 4 years/60000 km (37,500 mi) - whichever comes first, is the service interval for plugs.
The Cupra is my first petrol car for years and I’ve not replaced them yet.
Part numbers are here:
ignition coil<br/>spark plug<br/>impulse sender<br/>camshaft Leon/Leon 4 (LE) 2016 year Seat EUROPA 905050 spare parts
seat.7zap.com
If tuning the car, many people use the NGK Audi RS7 spark plugs as an upgrade.