In The Netherlands, its 5 gram per person max, and IIRC also 4 or 5 XTC pills (I can't quite remember). Though that information is from '00s. It might've changed, I don't know.
For weed, the law states a maximum of 5 grams (or 5 plants, if you grow them).
For any 'hard' drugs, such as XTC the legal limit is 0. But if you get caught carrying an amount that is small enough to be considered for personal use, the police usually won't act on it.
If you carry larger quantities than what is considered personal use, it is usually confiscated and you risk a fine (but only if the police has indication that you are selling the drugs).
The weird thing is that the 5 gram rule also counts for the weed dispensary (which are known as 'coffee shops' in NL). So even though it is legal for them to sell the cannabis from their shop, they can't legally buy it from the grower. They aren't even supposed to have more than 5 grams in stock. The growers that supply the shops must also do this illegally, and they are constantly playing cat-and-mouse with the police.
Clearly, the law is ambiguous and both police, growers and dispensary owners are encouraging the government to fully legalize cannabis.
We had odd ways to go around the police. Once in a year they'd show up, in numbers. Some kind of yearly ritual. Our main trick was to accidentally throw down a trash bin in such a case so that the "refiller" (trusted middle man with social skills who sat between the growers and me) would know that he shouldn't show up. All the stash was stored just 2 floors above the shop in a civilian apartment floor. He'd hear the noise through the heating pipe.
I'm glad I quit because, quite frankly, I didn't have the social skills for this job (autism, lack of balls for the semi criminal cirtcuit, street slang, etc). I felt my co-employees and boss were more focused on sales than on proper guidance of tourists. I've refused to sell people (especially spacecake) based on maturity/knowledge signals I picked up. Though that wasn't the main reason why I quit I'm glad I don't have to feel guilty for that anymore (because there's always a losing party to such decision).
From the article, it sounds like Luxembourg is the same:
"those who break the more generous laws will be hit with harsh penalties under the plan"