How many entities exist in the following document? write a number $[[$Registration form$],[$Student Name:$],[$Student ID:$],[$Major:$],[$Address:$,$Date:$],[[[$Course$],[$code$]],[[$Course$],[$name$]],[[$Course$],[$credit$]],$Section,$[[$Course$],[$date$]]],[,,,,],[,,,,],[,,,,],[,$Tot,redit,,$]]$