Queues should have an ideal size based on your latency requirements.

Say, you have 100 requests coming in per second. Each consumer of your queue requires 0.3 seconds per request. Now, you can optimise for the number of consumers. Would you choose 40 consumers, then the probability of serving the request with an empty queue in 0.3 seconds would be 17% (and > 0.3 seconds would be 84%). However, the total waiting time per request would be 0.1 second.

With 80 consumers, the probability of an empty queue would be higher at 58%. However, most of those consumers are doing nothing, while your waiting time is only reduced by 0.1 second! Only a 30% improvement.

(edit: probability of empty queue in second example is higher, not lower)