According to Official Guinness Records,
The largest football trials was achieved by Peneirinha Gillette with 5,119 participants in Rio Grande do Sul, Brazil from 8 September to 18 November 2012.
The trials were for children aged 8-9 and 10-12 in two states, Santa Catarina and Rio Grande do Sul - however for the purposes of the record only the children aged 10-12 in Rio Grande do Sul were counted. The largest participation in a single city was 1,988 children in Porto Alegre the state capital. At each city a handful of children were selected for the finals with one winner from each category chosen. The winners were awarded a place at the 'Football School' of their choice and a trip to Europe to train with a world famous football club.
For a complete list of 2012 records, please visit 2012 Guinness Records in Brazil.