You cant specify multiple recipients in the right hand side of the recipient_bcc_maps
. So
source@domain.tld rcpt1@domain.tld,rcpt2.domain.tld,rcpt3@domain.tld
is not possible. But The BCC address is subject to virtual alias expansion (and aliases(5) expansion for any resulting local recipients). So
#/etc/postfix/main.cf
#...
virtual_alias_maps = hash:/etc/postfix/vmaps
#...
#/etc/postfix/recipient_bcc_maps
source@domain.tld rcpt@domain.tld
#/etc/postfix/vmaps
rcpt@domain.tld rcpt1@domain.tld,rcpt2.domain.tld,rcpt3@domain.tld
should help you to achieve it.
Ref: Postfix Mailinglist