I perfectly understand your question. I'll rephrase my answer.
becasue you do not need neccessarily need a welcome mail for each list have a different one for each list the welcome mail is attached to the list. And you are right, if you have six lists and you want to have a welcome mail for each of them (identical or not) you need to attach a welcome mail to each of those lists. For the Starter and Essential version this is how it works.
The mass action @irfanmoosani suggested is a manual action and I guess you want the welcome mails to be send automaticly. Acymailing Enterprise comes with a real timesaver called Automations.
If you have the Enterprise version and you want one identical welcome mail for several lists you can use Automations for that. In this case you do not attach a welcome mail to each list. Instead you create an Automation that will be triggered as soon as a user confirms a subscription and it will perform an action for you, like sending a welcome mail. You set this nmail inside the Automation and you also set to which list it does apply. That way you only have one welcome mail to maintain.