This is a cute little circuit that feeds the 24 VAC to the light bulbs.
It's "hard" (low resistance) in the sense that the triac is on most of the time.
It's "soft" because the rest of the time it's just a 470 ohm resistor.
A 50 mA bulb will cause a 12 V drop when the triac is off.
(Ok, the triac is off only in the low voltage part of the AC cycle so it doesn't generate that much, maybe a volt or two.)
The thermistor will reduce the current if too many bulbs are on.
There's no reason that a dozen bulbs can't be on at the same time.
The triac also can be a dimmer for all the bulbs, it's just a question of the voltage of the two zener diodes.
Edit: Graphic corrected, added sensing, added 24V supply