CSV Importer documentation states in error you can import to multiple projects

https://confluence.atlassian.com/display/JIRASOFTWARESERVER073/Creating+issues+using+the+CSV+importer has a section on importing issues to multiple projects via CSV, but this cannot be done via the Issues -> Import Issues from CSV -> Bulk Create Setup screen as stated in the article as you do not have the option to map Project Name and Project Key. You only have these options with importing from CSV as an admin via the System -> External System Import -> CSV -> File Import screen.

It appears as much of the text of this article article was copied over from https://confluence.atlassian.com/display/AdminJIRAServer073/Importing+data+from+CSV, which describes how to import data via this latter method.