How many actors appeared in it?
SELECT count ( T1.Name )  FROM actor AS T1 JOIN musical AS T2 ON T1.Musical_ID   =   T2.Musical_ID WHERE  t2.year  =  1986 and t2.category  =  'Best Book of a Musical' and t2.Result  =  'Nominated'
count actors = COUNT(actor.Name); appeared in it refers to joining actor with musical based on the condition: musical.year = 1986, musical.category = 'Best Book of a Musical', and musical.Result = 'Nominated';