To add multiple IRS Schedule Cs:

• go to Business tab; then Continue
• choose Explore on my own or I'll choose what to work on  (if it comes up)
• then Business Income and Expenses
• then Profit or Loss from Business, click Start or Update.  
• you should see the Business Summary page that lists your businesses, 
• right below the list you can Add Another Business.

 

Which version of Turbo Tax (for 2017 taxes) is able to produce MULTIPLE Schedule C's in the same return??

You need to use the Online Self Employed version or any of the Desktop programs have schedule C, but you  will get the most help in the Desktop Home & Business version.