Is there any maximum current query number in DNS?

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Hi all!
I am managing 2 DNS with load balancing.(Our systems have 512M Memory, 1cpu,
9G H/D)
By the way one day, dns queries were not resolved.
So I put one more system into load balancing group.
After that resolving is OK.
So, my question is
Is there any  limited maximum current query number in BIND 8.2.2.
Could you answer my question?
Thank you!!