Can anybody explain to me why my emails are disappearing after a week I have an iPhone 12 pro max. And then could anybody tell me how to fix it. It’s driving me crazy

In that case sign into your email account on a computer browser to see if they are indeed there and just not on the phone. If they are missing there too then contact your email provider for help with their management of emails.

If they are on the computer but not on the phone then double check again for Mail Days to sync, then remove the account and add it back.

Some email accounts have a "Mail days to sync" setting in:-

Settings, Mail, Accounts, tap the account in question.