The issues is Studio Canal don't see to communicate there's no page on there site detailing there steelbooks when they came out or when there releasing new ones so when new ones are coming, for me i only found out about Porco Rosso So that leaves 9 films with no steelbooks however there is also the Italian steelbooks which cover ones studio canal has made including the other 9 Studio Canal still need to make Up to now there have only made 13 (and The Red Turtle but this isn't a official studio Ghibli film)
So if you collect Studio Ghibli steelbooks the Uk ones are made by Studio Canal there are 22 official studio Ghibli films