I am "constantly" running out of closevotes. It used to happen just once or twice a month, but I've now I've hit it for the second time in a week. To be clear, I am referring not just to running out of reviews, but total closevotes; I run through not only the close vote review queue as far as I can go, but also can't vote to close questions I encounter in the low-quality queue or from organic browsing.
I have been searching and searching MSE and MSO on the basis for the limit, which is theorized to be an anti-bot in purpose. I can't locate a definitive question, just complaints that the limit should be raised.
I do see references to SO having a limit of 40 or 50 closevotes, rather than 20. This may be a special exception for SO, since their review queue numbers in the thousands rather than the low hundreds, but if this limit can be adjusted, I'd like to petition for it for EL&U as well. While our queue is smaller, we also have correspondingly fewer reviewers, and while we have very diligent moderators, during holiday periods the number of pending reviews still grows noticeably.