The numbers above are just guidelines. If you hate doing laundry, you'll probably want at least one top for each day of the week. After all, close fitting shirts like t-shirts and tank tops get dirty and stinky fast. So, it's perfectly okay to have 7-8 shirts.
The idea is to just keep things manageable. You don't want to end up with a month's worth of the same type of shirt in your closet.