A long time ago I've created the site key and secret for recaptcha v2 in the google cloud console.
Now all I can find there is reCAPTCHA Enterprise API with an ENABLE button,
the recaptcha on my site still works, but I can't find it's secret and key values on the google console.
I wanted to add another site key
You can find the secret key which was created long back by following this Find a legacy reCAPTCHA secret key. Under the integrations, you can find the secret key. Note that if those keys are deleted then you cannot recover deleted keys. In order to check the keys you need to Enable recaptcha Enterprise API.
There is no limit on the number of reCAPTCHA keys that you can create for a Google Cloud project. It is best to create one reCAPTCHA key per website. You can create a new sitekey by following this document.
You can also check this blog by Digamber for more information.
I recommend you to use recaptcha enterprise only as it helps/ protects your website from fraudulent activity, spam, and abuse without creating friction and this is having more features Compared to v2 and V3.
The different features in each version of reCAPTCHA can be seen in this documentation and Compare versions documentation.