The alphabet proper with the most letters is the Georgian alphabet with 33 distinct letters. The Georgian Alphabet is primarily used in the country of Georgia.
The abjad alphabet with the most letters is Sindhi with 51 letters. Sindhi is used in Pakistan and India.
The abugida alphabet with the most letters is Khmer with a total of 74 letters. This alphabet is used in Cambodia, Vietnam, and Thailand and in Australia and the USA by Khmer speaking people.