Is it possible to limit concurrent aggregations in Mongo? When clients makes certain requests, our service sends aggregation queries to Mongo. Some of these aggregations take longer than others.

Usually this is fine, but occasionally multiple heavy aggregations will be requested at the same time. In the worst case, this causes Mongo to become unresponsive, and other requests (such as inserting new documents) fail with a timeout.

Is there any way I can limit the number of aggregations that Mongo will perform in parallel?

Ideally if 8 aggregations are running and a new aggregation request is made, Mongo would queue the new aggregation until one of the others has completed.
Answer is simple, NO. Not in mongodb. But you can always do it in your own application. Just create "lock" collection where applications mark they will to do aggregation. Ticket system for queuing.