Also there's some functions you use all the time so you'll get to know them quickly (ex: find, remove, push, mapcar, assoc) and others you'll probably never need (ex: tagbody, prog, progv). In short you only have to learn a fraction of those 978 to be able to make something significant.
You can consult http://www.lisp.org/HyperSpec/FrontMatter/Chapter-Index.html when you need deeper knowledge in a particular topic.