To import contacts with multiple tags in one field, list those tags in a single column in your CSV file and separate them with a comma.
Your single cell can look like this: Ice Cream,Cookies,Cake.
In this case, it does not matter if you choose to use spaces or not in between each of these tags - each of these items will appear as a separate tag for your contact.
Then on the import page you will want to map your tag column with the "Tags" field.