They would be a bit dim. 12 bulbs across a 240V mains means that they are each 20V. Also, running even six in parallel would mean a lot of bulky wire. Instead, I use a variac (variable output mains transformer).
Note that modern incandescent bulbs (if they haven't all by now been succeeded by LEDs!) have a fail-safe loop of wire around the filament supports. If a filament fails, the supports spring apart, and the wire loop provides a S/C across the bulb - so the remaining bulbs in the string stay lit (albeit a little brighter).