I'm assuming you mean a quadrature encoder, with two digital outputs; so it counts 00, 01, 11, 10, 00, ... in one direction, and the reverse in the other.

With such an encoder, the maximum speed occurs when you get one new code every cycle time. So if e.g. the encoder generates 48 codes (which corresponds to 12 rising edges on a given output) per revolution, and the cycle time is 1 ms, then the maximum speed is one code per cycle, or one code every 1 ms, or 1/48 of a revolution every 1 ms, or one revolution every 48 ms. That's about 20 rotations per second.

I'm not sure what you mean by a 24 V encoder; but if you have two quadrature digital outputs going from 0 V to 24 V, then you could just use two voltage dividers to make 0 to 5 V logic levels for the micro, and connect those to two digital inputs.

An example is attached, with one possible way to decode the quadrature.