There is no limit if not the price! Moreover, truffles, as all mushrooms, are made of 80% of water, therefore they do not have many calories.
Generally speaking an average of 8-10 grams of white truffle per serving is considered enough to savour it.
The truffles can be tasted on one course or throughout the entire meal. In this case it is suggested to ask the restaurant manager to bring their display of truffles to the table.