Assuming you are talking about a holiday home (as there is no CGT to pay if you have been resident in your home for the majority of the time you have owned it, I believe) it is 20% of the PROFIT shown.
So that is the selling price minus the amount paid as declared in the deed (NB this might not be the actual amount you paid!) and any professional fees and renovation costs (which need to be invoiced).
Hope that helps but I would recommend seeking the advice of a notary to confirm actual amounts.
Also the Italian tax office has some info in English here;
Agenzia delle Entrate - Home page english version