According to Official Guinness Records,
The largest gathering of people dressed as astronauts is 257 and was achieved by Sea Mills Primary School (UK) in Bristol, UK, on 10 November 2017.The attempt took place to raise awareness of STEM subjects (science, technology, engineering and mathematics) and raise money for The Prince's Trust.
For a complete list of 2017 records, please visit 2017 Guinness Records in United Kingdom.