It is possible you have an email account with "Notes" enabled on your iPhone, but don't have the same account set up on your Mac.
On your iPhone, in the Notes listing, tap "Accounts" in the upper left corner. Mentally review those accounts.
On your Mac, click the Navigation icon that shows two lists next to a box, and makes sure the accounts are the same.
If the accounts aren't the same, setup the missing accounts on your Mac in Mail Preferences. Also, if you have an Note "Account" on your iPhone that is named something like "On my iPhone", transfer those notes to an account that syncs. You can accomplish that by, tapping the Share icon in the note, tap "Copy", tap "Notes" to view your list of notes, tap "Accounts", tap an account that will sync to your Mac, tap the "+" icon, tap and hold, paste the note that. (Hopefully you don't have too many notes that need that action).