first of all, you have to create a category for each of your author (you can name the category with the name of the author, just to make things easier).
then in your main index template, where you want the "number of post made by this author" to appear (i suppose after the name of the author itself), add the following:
CODE
<MTEntryCategories><$MTCategoryCount$></MTEntryCategories>
this will show the total number of posts in the category wich the current post belongs to (ergo, the one containing all of the posts made by that author).
...and that's it!
only one little shrewdness: pay attention to assign every post only to the category of the author (not using "multiple category"), because this would make a mess of the total posts' count!