This is a bad idea and the situation you compare it to (Police quotas) are equally as bad of an idea. I appreciate the intention behind it; you would like to see merit freely flowing to the posts you believe to be deserving of merit. However, merit quotas for the sources would change everything for the worse.

Currently there is no obligation for active you are, how much time you spend on the forum, how many posts you read or how you dish out your trust/merit. If we enacted a quota for merit sources, then it would be necessary that a merit source is active X amount of time per day, reading Y amount of posts and meriting Z amount of them. This would inevitably promote spamming beyond belief; if there are quotas and you can only find spam posts, you would inevitably be "forced" to give a spam-post a merit just to keep your merit source status and keep up with criteria.

It would serve to benefit those that don't deserve merit and it would put extra strain on merit sources, which would lead to them doing a worse job at distributing their merit, not a better job.

Out of 10 posts there will be a "best" post, but "best" will be subjective and the "best" out of those 10 could still be horrible and not worth a merit.