This is a "how long is a piece of string" question!  The accepted wisdom is that a Lurcher is a form of sighthound crossed with another working type of dog.  A typical example is a Greyhound X Border Collie. This gives you a hunting dog with speed and stamina.

However, a Lurcher is a TYPE of dog and not a breed.  There are many different crosses which are classed as lurchers and some of the most common are :

Saluki x Greyhound
Bull Terrier x Greyhound
Bedlington Terrier x Whippet
Bearded Collie x Greyhound
Border Collie x Greyhound
Border Collie x Deerhound
Jack Russell Terrier x Whippet

But there are endless variations- and if you have a very active lifestyle, you may want to consider a lurcher rather than a Greyhound as the terrier, collie or saluki in them should enable them to keep up and indeed thrive on the exercise. On the whole they have very similar traits to a Greyhound and many are just as laid back and happy to sleep the day away, or on the other hand they may need the exercise and the mental stimulation of a collie.  All dogs are individuals and again you will be given advice from Scottish Greyhound Sanctuary on whether a lurcher would be suitable for your lifestyle.