PPR for rotary encoder

[engkeat]

in my design, the motor is required to lift up/down a base with a constant speed within 5-8rpm, i m using incremental encoder as the speed sensor and PIC-16f877A to control the pwm, the motor is directly connected to the base as well as the encoder, can anyone guide me in determine the required ppr ? do i need any interface between the encoder and the PIC? thanks