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.
Everything that EthanB said is on point. Abuse in many forms has been rampant in this forum, let's not corrupt the merit system by promoting something that would lead to its misuse. There is a reason why merit sources are appointed by the moderators of this forum and it's because they know what quality is. You don't have to force them to give out merits just because they need or obligated to reach a quota. What will the point be if out of the 10 posts you submitted the "best" is not sensible at all? We all have freedom here in this forum, let's not suggest things that would not lead to the betterment of this forum.