According to Official Guinness Records,
The largest plastic duck race involved 205,000 ducks racing down a 1 km (0.6 mile) stretch of the River Thames, UK. The event was organised by the Great British Duck Race (UK) on 6 September 2009. The event was organised to raise money for a range of charities, including NSPCC, managing over £100,000 in contributions. Each duck cost £2 to enter the race and the owner of the duck first to cross the finishing line receiving a prize of £10,000.
For a complete list of 2009 records, please visit 2009 Guinness Records in United Kingdom.