New thermostat wire: maximum number of wires?
Since your question relates to "future proofing," it all depends on where you think thermostat technology is headed.  These days, I would not ever install less than 18/7 thermostat wire if I had a choice.  This also happens to be the number of wires that you need for your setup if you tie your humidifier common to the system common and use the ecobee3 "1-wire configuration" for the accessory setup instead of using the 2-wire configuration that assigns the accessory terminals as independent dry contacts.

The future is probably digital.  If you want to cover all likely residential possibilities then pull two cables, an 18/8 thermostat wire and a CAT5 (or better) network wire.