Why do I have only one iPhone backup on my Mac?

Should there not be several?

No, a backup backs up the changes that have occurred on your iPhone since the last backup.

If you are asking about creating a history of backups, this is possible but it must be done manually and will be quite time consuming....and....will take up a ton of space on your backup computer or iCloud.

no they override it to not fill up your computer storage, today it's not uncommon that an iphone has more storage than a computer