I just backed up my iPhone but don’t see all of my contacts in iCloud. What am I doing wrong

This is often confusing to many; but essentially the backup only backs up data which is NOT already saved and/or sync’d off-device by other mechanisms.

Excerpted from:

What does iCloud back up? - Apple Support

“Your iPhone, iPad, and iPod touch backups only include information and settings stored [only] on your device. They do not include information already stored in iCloud such as Contacts, Calendars, Bookmarks, Notes, Reminders, Voice Memos”

Your contacts are either separately stored and backed-up with your e-mail application; while any iCloud Contacts are continuously synced to iCloud separately.

You can confirm it’s setting is “on” at:

Settings -> AppleID -> iCloud -> [Show All] -> Contacts

As you can read in the article linked to above, backing up to iCloud doesn't back up your contacts.

What account are your contacts associated with? iCloud? Google? Yahoo? Something else?