Why max connection is 250

Hi,

I install mosquitto on my mac air, I start mosquitto.

and bellow is my step:;

1. run on shell : mosquitto_sub -t "\$SYS/broker/clients/active"
    I got 1 active client.
2. then run bash on shell:(try to simulate 249 clients' connections)

#!/bin/bash
c=1
while [ $c -le 248 ]
do
mosquitto_sub -d -t t -k 9000 &
 (( c++ ))
done

3. so far, everything works ok. and I can publish message to all these clients.
4. then I try to simulate one more client.
          mosquitto_sub -t t
    And I can see there is 250 active clients from the log.
5.then I try to publish message to all these 250 clients,
         mosquitto_pub -t t -m 12
   I got "Error: The connection was lost."

I have tried many times, got the same result, can anybody help me? actually I want to do a pressure test to know how many connections mosquitto can host. any suggestions?

Thanks.