whatsapp backup from different phone number but same gmail account How can I restore my old chats on whatapp from different phone number but same gmail account?
WhatsApp keeps its chats on your internal storage in the folder /WhatsApp/Databases. There might be a large number of files, the latest being the one with no date in it (msgstore.db.crypt12, probably). 

I'm not sure if the phone number being different matters if you have not enabled WhatsApp's 2FA, and if you have enabled cloud backups to Google Drive as long as you use the same Google Account you should be okay.