First of all, it is “PIM(personal iterations multiplier)”, not “PIN”. It has nothing to do with PIN numbers. And it is not VeraCrypt’s fault that someone misreads PIM as PIN.
Second, I’m not sure whether VeraCrypt prevents users from intentionally decreasing the iteration by using a low PIM number. But if you’re unsure about what PIM does, it is best not to modify it. This (seemingly obvious) rule applies to almost everything; do not change something if you’re not sure about it. No software/hardware can prevent a bad OPSEC. At the very least, read the official documentation or simply google “Veracrypt PIM”, and you’ll get enough information in less than 10 minutes.
Third, setting the PIM to 999 is not obvious at all. But even when the PIM value is known to the attacker, bruteforcing with PIM of 999 is harder than default PIM. PIM literally means ‘how many times to iterate the key derivation function’, and higher the value, greater the calculations it requires.
Lastly, I suggest that you first go through the official documentation before asking basic questions. Might seem a bit long, but it’s definitely worth reading it.