I bought the premier at costco. I can download or use the CD. Can I file 6 tax returns in the CD or downloaded version?

When you use the CD/download you can e-file up to five federal and state returns.  If you have more than five for family members etc. the extra ones can be filed by mail.  The five e-file limit is an IRS limit.

FIVE Efiles  is the limit for the program you purchased ... all others will have to be paper filed.

If one of the returns qualifies you could use one of the IRS FREE FILE options : 

https://www.irs.gov/filing/free-file-do-your-federal-taxes-for-free