Thanks for your reply. My first point is that the displayed number would be factually worthless. After all, whether it says 500 or 25000, most of those players will be either substantially better or worse at the game than you. In other words, the actual number does not really mean anything. It also is not indicative of the chance that you will get into a match, because the amount of matches that the system has to make increases in linear fashion with the number of players queued up. As such, your chances of getting into a match are not better with 500 players queued up than they are with 25000.
If you insist on displaying a number, then you would somehow have to make it meaningful by specifying further criteria. The example I used narrows the number down to the amount of players in the queue that approximate your rating. However, the more accurate you make it, the more the displayed number will approach zero. In the ranked 4v4 queue, for example, the system tries to match a total of 8 players. It might take a while for the system to find 8 players of the same skill. Once a match starts, these 8 players are removed from the queue. This would reduce the number of queued players around that rating to zero, until new players queue up.
Depending on which option you choose, a display of the number of players queued up is either entirely meaningless or bound to be very low. If a gross estimation of your chances to get into a match is what you want, then I must furthermore point out that the game already contains such a thing. Namely, when you are in the queue, the game shows the estimated time until a match is found.
My second point is about the player interpretation of any kind of displayed number and possible consequences. Whether a number is high or low is a normative matter that is not provided by the facts. I might find 500 players in the queue high, or I mind find it low. 500 may be much higher than what I saw it to be yesterday, or far lower. In other words, a simple number is subject to interpretation and from that interpretation may follow encouragement or discouragement. If I find 500 players in the queue low, then that discourages me from queueing up myself.
Now, your suggestion appears to be motivated by the insufficient amount of players queueing up for DM team games. The to-be-displayed number of players in the queue would hence unlikely surpass tens or hundreds. The closer this number approaches zero, the bigger the chance that I will interpret this number as being low, the more likely I will be discouraged from joining the queue myself.
You attempt to refute my second point by arguing that the game already discourages players from joining the DM team game queue by not displaying any number at all. However, it is not correct to speak of discouragement here, for it is more accurately a lack of both encouragement and discouragement. In other words, the current situation is neutral, for the absence of a displayed number also entails the absence of player interpretations of that number. After all, if there is nothing to interpret, then neither are there interpretations of good or bad.
To display the number of players in the queue, then, is to allow players to actively interpret that number as good or bad. Given that this number may be low, in cases such as DM team games, there is the real possibility that it will discourage rather than encourage players to join the queue. If one player does not join the queue because the low number discourages them, then it is increasingly likely that the next player will make the same judgment. Consequently, displaying the number of players in the queue as per your suggestion is not a measure with a guaranteed positive outcome, but is equally likely to have a negative effect.
Given that the number is both meaningless and possibly counterproductive, I conclude that it is bad idea.