Meaning of server_names_hash_bucket_size

[Liang Jin]

I have seen the following error in nginx log on my server:

could not build the server_names_hash, you should increase
server_names_hash_bucket_size: 32

The problem was solved after I changed the setting to 128.

However, I am curious to know what the setting will actually do. If I
have many virtual hosts, I will need a larger number for the bucket
size, but how can I determine what would be a good number to set this
parameter?

Thanks.

-Liang