Sometimes Celebrity makes things unnecessarily complicated. This is one of them. On the M and S class ships there is a specialty restaurant called the Tuscan grill. It is an extra charge venue.
On the E class ships, there are four main dining room and one of them is called Tuscan. There is no extra fee for dining there.
That’s why it was important to know which restaurant/ship the OP was inquiring about.
The general answer still holds, you can order as many of each course as you wish. You can even order three appetizers and no main course, or two appetizers a salad and dessert. If you ask them to bring you two main courses, that’s usually just fine, too.