It depends on your provider/mail engine. No limit on mine, comma separates. Some systems allow you to set up a list of names then just refer to the list name when mailing.
If you're mailing to lots of people it's often wise to use bcc otherwise you're providing people with masses of addresses.
Er, ta canary, but if you don't know me, I'm a techno-duffer, so...my mail package is Outlook2013...can you help with that, re copies and er 'dividers'?
are they in your contacts list? if so just click on the name in contacts. Be a bit careful with long lists as some email providers can see it as a sign that the email is spam bit you should be ok with 6 or so
So I use MS outlook and Hotmail all day every day and send loads of emails to multiple people. I never have to add a comma or a colon or anything - it does it for you automatically. Enter the first name, ensure it's registered properly by becoming underlined, then add the next name etc. Outlook will add the spacers for you.