Building on Jazz's note, you can have one switch per axis as long as it is actuated both in both the + and the - direction. But you must have both directions protected! As I said above, the soft limits are only there as a means secondary safety and only work if your machine has homed (Like Clive says)

here is a photo from practicalmachinist forum showing a single roller switch that is actuated by rolls that mount to the face of the table: