You don't have to upgrade, but Odoo 12 is no longer officially supported (we support the latest and the two prior versions - so now that is Odoo 17, 16 and 15).

See https://www.odoo.com/documentation/17.0/administration/maintain/supported_versions.html

When we don't support a version we no longer investigate bugs or release security or performance improvements for that version - you are essentially "on your own" when it comes to issues you want help with - although some Odoo Partners support older versions.

Have you seen the Documentation at https://www.odoo.com/documentation/17.0/administration/upgrade.html ?

If you have other questions you can always run then by your Odoo Account Manager.