Categories is useful , the things is can we choose a category and then we click those emails let say 8 emails at once , so that those 8 emails is mark as choosen category , instead of one by one?
please advice 

Hi Fransnfun,
you can definitely put more emails into a category at once.
Just select all the emails using CTRL + LEFT CLICK and click all the emails you want in the same category.
Then RIGHT click one of the selected emails and then choose Categorize > Your category selection.

Best regards,