This is the maximum number of items on the mail queue.
Although the items on the queue are small (less than 10 kB each), the more items are waiting on the queue (as per the Currently on Queue value above), the more memory is allocated.
Theoretically this may lead to Out Of Memory errors putting the system stability at risk. Therefore a hard limitation is introduced to prevent this.
Even the largest Jira instances have normally not more than a few hundred items on the queue as an average.
If you experience this constantly, consider increasing the number of worker threads.