I seem to remember they had even more than that in the distant past too. The deaths have been for a variety of reasons. Some of those chimps that died were very old, like the chocolate female Coco(58) one of the most recent deaths. Two died in a tragic heating accident. At least one died from the a heart attack(?) during the process of mixing them all together. That process may have accounted for other deaths too as not all have been made very public I don't think.
Twycross are certainly in a better place with this single group, living in proper modern housing, even if it is the lowest number they have( probably) ever held since the earliest days. But with mainly middle-aged and older hybrids, they need a fresh infusion of some younger females if it is to prosper in the future.