Six bells commence tolling together and toll at intervals of 2, 4, 6, 8, 10, 12, minutes respectively.

Six bells toll together at intervals of 2, 4, 6, 8, 10 and 12 minutes, respectively.
Prime factorisation:

$2=2$

$4=2 \times 2$

$6=2 \times 3$

$8=2 \times 2 \times 2$

$10=2 \times 5$

$12=2 \times 2 \times 3$

$\therefore L C M(2,4,6,8,10,12)=2^{3} \times 3 \times 5=120$

Hence, after every 120 minutes (i.e. 2 hours), they will toll together.

$\therefore$ Required number of times $=\left(\frac{30}{2}+1\right)=16$