how many spaces would be
already filled by the items in our present fuse box and how many extra
spaces would there be?

You'll use the same number of breakers as the number of fuses you're using
now.

True, but on a breaker box more space is better. What I can't say for
sure is how many arc fault or ground fault breakers that they might
want to put in. Those take up a lot of space. More space is
beneficial because then you can put all your applicnaces on separate
breakers, and then your toaster oven and microwave on separate
breakers as well, etc.