druid.server.http.numThreads is a configuration that can be set both on brokers and historicals.
What happens if:
numThreads is set to higher value on broker compared to historical
numThreads is set to higher value on historical compared to broker
In general the broker will have a higher number of http.numThreads than the historical nodes. This is because a single broker will communicate with all the historical thus needing more threads.
If you set a higher value on the historical than the brokers nothing bad will happen, but it probably will not utilize all of the threads.
The formula to determine the default value of druid.server.http.numThreads is:
max(10, (Number of cores * 17) / 16 + 2) + 30