I've been wondering this too. As I understand it, BLIK codes are generated on the back-end, so I imagine they have some clever anti-collision measures in place. What I know is:

- The TTL of the code is variable; on some days I've noticed it to be as low as 60 seconds, on others around 3+ minutes. Not sure if it depends on the type of transaction or time of day.

- After entering the code in charging widget/terminal, or giving it to a merchant, you still get a screen on which you need to explicitly confirm the transaction; it displays the amount and name of charging entity, so this would presumably reduce the impact of possible collision.

- Sometimes the codes generate instantly, sometimes it takes a few seconds; I always assumed it's network connection lag and/or usual webshit performance issues, but it would also be consistent with an anti-collision measure - if you run out of 6-digit codes, wait a second or two, some will free up.

- Not once I've heard any report or rumor about a collision.

IIRC a few years ago I saw some store asking for 6 or 8 digit BLIK codes, I guess the latter was how they were planning to expand from supporting just Poland to supporting whole EU. But that effort seems to have died out.