I made this almost a week ago, halved the recipe and have just had the last piece today - it made 6 serves for me.
I didn't have Amaretto so used my home made
coffee liqueur. I used as many biscuits as I needed for my 20cm square tin but next time I would dunk them in the espresso coffee mixture instead of spooning the mixture over the biscuits once in the tin. As you can see from the picture in the recipe book, the bottom layer takes up the hot liquid but the top layer doesn't do as well.
My mascarpone mixture seemed quite runny (I did make sure I halved everything) for the first 3 days but then it thickened up to what it should have been on day 1.
I would definitely make this a day or 2 in advance and if halving it again, maybe use a vessel a little bit smaller than 20cm square so that there would be more mascarpone between and on top of the layers.
It was delicious and I would definitely do it again.