Which sentence is the correct one?

A number of = কিছু সংখ্যক, এর পর Noun ও Verb plural হয়। তবে The number of এর পর Noun plural হয়, কিন্তু Verb Singular হয়। Many এর পর Noun ও Verb plural হয়। কিন্তু a many এর পর Noun ও Verb Singular হয়। 
Correct Sentences:
ক) A number of students were present in the class. 
খ) The number of his mistakes is immeasurable. 
গ) Every year a lot of children die of cholera.