They *can* go up to 100mbps with copper (VDSL), and they have some VDSL plans (I think up to 40mbps) available in some of their coverage areas (VDSL typically allows 50 to 100mbps up to 1km, but AIrtel aren't that ambitious).
24mbps (20ish in real life) is available with
ADSL (2+), which is what they are probably using to deliver their 16mbps plans, whereas 8mbps is the max you can get only if your DSLAM has old ADSL equipment in it.
Standard attenuation calculations indicate that ADSL2+ would sync at and be able to deliver 16mbps comfortably if the copper loop is anything up to about 1.2km, whereas original ADSL should give a solid 6-8mbps up to about 2km (roughly). Assuming the line is in "decent" condition, of course, but most of my experiences with DSL was using 50 year old copper, so...
If you can't get the 16mbps plan, it's probably because the DSLAM is old and they haven't upgraded it (DSL equipment is about US$100 or INR6k per port, so, you can probably guess why).