I've lost most of my notes on "Notes" in my iPhone 8 plus.

Hello and welcome to Apple Support Communities, 4226295,

We understand you've lost some notes on your iPhone 8 Plus. We'd be happy to provide information to help you address this.

Have you recently removed an email account from your iPhone? It's possible this has triggered the missing notes: If you're missing notes on your iPhone, iPad, or iPod touch - Apple Support

"Check your account settings

If you store your notes with a third-party account or email service, like Gmail or Yahoo, check your settings:

1. In the Settings app, tap Mail.
2. Tap Accounts, then tap the email account that you want to check.
3. Make sure that Notes is on.

If you use Notes with more than one email account, just repeat the steps for each account. If your account is missing, learn how to add an email account to your device."

Regards.

Our pleasure, 4226295. Have a good one!