How do I make my iPhone 8 ring more times before it goes to voicemail?

Contact your carrier to see if they have an option for this.

Most carriers have a user site that will allow customers to select the number of rings....for example.....between 1 to 8 before a call goes to voicemail.

That option would be controlled by your cellular provider. Contact them to determine if changes are possible.

The number of incoming rings on your iPhone is determined by your carrier. If you want to extend it, you should contact them.