How to retrieve deleted emails on my iPhone?

Nor iCloud Backup or computer backup includes Mail data. See https://support.apple.com/en-us/HT204136

If you deleted by accident they all should be in a Bin folder.

If gone from bin too, contact your Email Provider to start (if any) data recovery process.

Your other options would depend on what other devices have access to the account where emails are missing. E.g. if you have a Mac where you use Mail app too, if you have Time Machine backup running, you might be able to get the emails from the backup of your Mac; it will require to put back emails into Mailboxes on the account to sync through email server back to other devices.

Is it set to backup to Icloud - or do you have a backup on your computer.

If you have a backup from a couple of days ago, you can restore it back to that (but will lose anything added since then)

If you have no backup, then they are gone (subject to second link below). See:

Restore your Iphone from a backup

See also

Macworld - how to recover deleted emails

Chances are when you deleted them they deleted from your mail service and you'd be very unlikely to get them back. Go to your mail service on your computer. Are the emails there or are they gone? If they are gone, they are gone as email is not stored in the cloud. It is synced with your mail service.

You CAN however contact your mail provider and let them know you accidentally deleted mail and ask them to return them to you. It can take a couple days, but I accidentally deleted about a weeks worth of email from Yahoo mail and I asked Yahoo to restore them, which they did.

You can't.