According to Official Guinness Records,
The most people singing a national anthem simultaneously is 509,261, achieved by Shree Khodal Dham Trust (India), in Kagvad, India, on 21 January 2017. The record was organised as a part of the opening ceremony of the temple built at Kagvad by Shree Khodal Dham Trust. The event attracted close to 1 million people according to the auditors, but only the above number could be accommodated in the record attempt area.
For a complete list of 2017 records, please visit 2017 Guinness Records in India.