According to Official Guinness Records,
The most naked people on a theme park ride is 102, set by members of the public on Green Scream rollercoaster at Adventure Island, Southend on Sea, Essex, UK, on 8 August 2010.
40 members of the public could go at one time, due to the number of seats available on the ride. The rollercoaster went 3 times to accomodate them all. The particpants raised over £22,000 for the 'Bosom pals' appeal, which is for the breast cancer unit at Southend University Hospital. Good media coverage.
For a complete list of 2010 records, please visit 2010 Guinness Records in United Kingdom.