There is no connection number limit (except for OS limits on number of
file descriptors and so); bu 65 replicas re-syncing simultaneously, with
operations that may require hours, will eat up all threads if configured
as the default. If you need to have so many replicas, you might
consider unloading the master from bulk search load, dedicating it to
centralizing writes, and configure it with lots of threads, so that it
can simultaneously deal with syncs and writes (e.g. 8 threads plus the
number of consumers, to be conservative).