[Completely rewritten answer]
That seems fun. But why 65 keys ? with 64 keys it is easier.
You have to test the sensitivity of the hall switches with the magnets to know how far apart the keys must be. Perhaps they only work with strong magnets or perhaps a number of them are activated when the glove is near.
With hall switches, there are 65 digital signals from the sensors.
The most common way to create more digital inputs is to use shift registers. For example with the 74HC165. Nick Gammon has the best tutorial of the 74HC165 with daisy-chain.
There is also an example is with the CD4021 and the ShiftIn function, but the 74HC165 is mostly used.
The shift registers can be daisy-chained, they are made for that.
Your AH1807 has a open-drain output. That means you need 65 pullup resistors.
It is not possible to connect all the outputs and turn them on one by one, since it requires 150ms to wake up.
Another option is using multiplexers. It can be used for analog and for digital signals. For example with four 16-to-1 multiplexers you can create 64 inputs going to 4 pins. The best multiplexer tutorial is once again written by Nick Gammon.
With four analog multiplexers of 16-to-1 you would require only 4 pullup resistors after the multiplexer. I think it is better to use a resistor per hall sensor (65 pullup resistors).
When using reed switches (those tiny glass tubes, used in house alarms), they can be set in a matrix/grid with rows and columns and the Keypad library. For example 10*7 requires 17 pins. The Keypad library allows that multiple keys are pressed at the same time.