16.8m people in the UK have a Nectar card

"Nectar, whose partners include Sainsbury's and BP, has overtaken Tesco's Clubcard to become the biggest loyalty card in Britain.
The card said it had added over a million holders in the past year - taking the total to 16.8 million.
That exceeds both Tesco's card, with just over 16 million holders, and about 15 million Boots Advantage Card holders. The milestone comes as Tesco's card prepares to celebrate its 15th anniversary this month.
Nectar has added Homebase in the past year, giving it 14 partners and about 400 internet sites."
Source: This Is Money, 8th February 2010