Multiple Drop Back's

So we where talking about the multiple Drop Back's in Iyengar yoga (here) . A recommendation of twelve to twenty was related in the comments section and it was mentioned that Iyengar himself did 108 on his 80th birthday.

Twelve to twenty does seem a lot to Ashtangi's who tend to do three to five.

or do we....

I was just looking at the Mathew Sweeney's book and in his section on Back Bending he has ...

Drop back's x5

Handstand drop-over x5

Viparita Chakrasana x5

Final Drop-over x1

Setu bandhasana x5

= 21 back bends