The answer is about 100. That is, 100 or so on display in the Glasshouse at Wisley at the moment. Pink ones, green ones, cream ones, purple ones, yellow ones, deep red ones (some are nearly black) and mixed up ones. Some have leaves that are almost round, while others are toothed and jaggedy. Some are small, some aren't, but they are all spectacular in their own right.
Paul Atkinson has been looking after these Solenostemon plants (commonly called coleus). He keeps the plants to a round, bushy shape by pinching out the growing tips. And he's just re-propagated the entire lot. Don't ask him which his favourite is - there are too many to choose from.
Coleus are very versatile. They make very easy house plants (easy to propagate too) and are sometimes used in summer or subtropical bedding displays, and are especially vibrant in shady areas. Painted nettle is a good name for them, although they're members of the mint family - definitely no sting with these plants.
These bottom two are two special ones - 'Wisley Flame' on the left and 'Wisley Tapestry' on the right (double click on the image to see a larger version). You can come and see the display, maybe try to decide which is your favourite, in the Glasshouse at Wisley where they'll be on display until the end of October.