...RFC 2821 section 4.5.3.1 says that an MTA MUST allow at least 100 recipients for each message, but MAY elect not to allow any more than that. I choose not to, since this is one of the favourite tricks of the spammer.
To set the limit, create a /var/qmail/control/maxrcpt file which contains the maximum number of recipients you wish to allow. If you don't want any limit, you can either delete this file, or use 0 as the value.