According to Official Guinness Records,
The most people playing a mobile phone game simultaneously is 1,080 people and was achieved at an event organised by Vivo Xplay6 (China) in Hangzhou, Zhejiang, China, on 26 August 2017.1,100 participants took part in the attempt but 20 of them were disqualified for not meeting the requirements. The game they were playing was 'Honour of Kings', a fantasy role-playing "multiplayer online battle arena" game.
For a complete list of 2017 records, please visit 2017 Guinness Records in China.