The rather surreal answer seems to be 5,440, according to media reports on Cadbury Trebor Bassett's Get Active voucher scheme launched this week.
As widely reported, the chocolate manufacturer is launching a £9m campaign to persuade children to buy 160 million chocolate bars in exchange for 'free' sport equipment.
Now, as the company digests the resulting acres of negative news coverage, it has defended the scheme to PRWeek, saying the controversy was anticipated and that Cadbury is in this for the long haul.
The real mystery is why, if this level of backlash was anticipated, Cadbury decided to proceed with such an ill-conceived and unimaginative piece of cause-related marketing.
Did the comms department advise about the potential for media calculations on how much fat would have to be consumed, only to be overruled? Or, more worryingly, did they fail to spot quite how delighted the media would be with the glaringly obvious irony of encouraging children to eat chocolate to fight obesity?
To a great extent, Cadbury has become a victim of the increasing fashion for such CSR initiatives. Companies are now labouring to reconcile the increasing government and public pressure on corporations to prove their credentials as responsible members of the community, while still meeting their responsibilities to shareholders in a very tough market.
In this context, the scheme must have seemed like a logical if rather unsophisticated solution. As Sports Minister Richard Caborn has pointed out, the obesity problem is created not by increased calorific consumption but by lack of activity.
But it doesn't make nearly as good a story as the fact that you need to buy more than 5,000 bars of chocolate to earn the equipment to play a game of volleyball. A little more cynicism at Cadbury might have saved a lot of red faces this week.