How 'safe' is a 12-digit alphanumeric key? To Access my data over a Portal i got a 12-digit alphanumeric key like:

VGW8-U98T-TXLV

How safe is this key? I don't want to know the chance to win with brute force. I want to know there are several standards to calculate keys like this? It is the one and only way to get access / safe my really personal health data. The portal can easily reached by a website. 

I can't tell more informations about how the key is generated or used. 
Maybe I'm a bit paranoid, but to get access with just these key and nothing else (e.g. birthday as second check) seems a bit easy. Even if the key give a lot of combination for a brute force.
Well, you have $(26+10)^{12}$ choices if the characters are truly random and independent. That's how we commonly judge security in these situations.  So that's $4.7 \times 10^{18}$ alternatives, or 62 bits of security. Hmm, not bad but we commonly refer to at least 128 bits of security on this site. However...

The real security question here is what happens if the wrong key is entered? Consider that a hole-in-the-wall machine only uses 4 digit PINs, but it gets angry when the wrong PIN is entered three times. What does your portal do? It would be really really bad if it just accepted incorrect passwords ad infinitum. Some form of lock out would be in order, or   a  delay (exponential) after each failed attempt.

There are standards/libraries for token generation such as the secrets module in Python with ad hoc security parameters. It's difficult to progress this further in light of the above uncertainty and your minimal revelations about the portal.  

And to the counter, any password like your's is almost certainly going to be written down.

And I'm assuming that your records are encrypted in some secure form, rather than directly  using this password without an intermediary key derivation function.