Why bother with the ATtiny at all? It's overkill. All you need is a Hall sensor such as the US1881. It can easily drive a few metres of cable to the Arduino, although you may need to use an external pullup resistor instead of the internal high resistance one.
As for the number of magnets to use, the US1881 switches on when it see a N pole and off when it sees a S pole. So you need at least 2 magnets. More magnets will give you faster update of the speed reading at low speeds, but they need to be alternating N and S poles.