The brown number appears next to the review button in the top bar only if there are at least 3 items to review. I would pretty much prefer to show this brown button whenever there is at least 1 item to review1.
Why? We do not get so much traffic as other sites, and we do want to get more people involved in the review process. Currently, we have probably got some people who (I suppose by a script) "click F5 every second on https://tex.stackexchange.com/review to know about any new reviews as soon as possible". While I don't mind them doing it2, I think it would be healthy for the community if this got changed.
A second thing is, especially in First Posts, Low Quality and Reopen queues, it makes the best sense to act on the posts quickly; since the queues are empty almost all the time, this is not the case: Users do not notice there's something to review until there's at least 3 things to review.
1 I'm sorry, I don't provide a screenshot, there's currently only 1 item in the queue and the button is therefore not shown.
2 Given their reviews are of very high quality.