Jeshii Heya, I'm not sure I understand what you mean by Privilege won't be valid until the stockpile reaches 50.
I don't think translators or translation groups will put the novels they are translating as a part of the Privilege program if they don't have at least more than 50 chapters in their stockpile. Because it's just too risky. So, if the highest tier that you've been offered is 50 advance chspters for a translated novel, then rest assured that you will always be 50 chapters ahead of normal readers for that novel (unless, of course, the novel is almost completed and the total amount of the remaining chapters for that novel doesn't add up to 50 anymore). If they don't give you that, then that's a breach of contract between a seller and their customer.
Original novels are a bit different. I don't think any original authors here could offer 50 advance chapters for their privileged readers. Simply because they don't have that many in their stockpile (if they even have a stockpile). Writing a story is different than translating one. It just takes more time. If they participate in the Privilege program, then the highest tier that these original authors can offer is probably 5-10 advance chapters.