Question

Why is the AC supply in multiple of 11 like 110V, 220V etc?

Answer 1

First of all, there are exceptions to the observation that the voltages are all multiples of 11
The voltage level started at 110 V before diversifying into 120 V, 220 V and 240 V for household usage. The voltages for transmission and distribution that are multiples of 11 are 11 kV, 22 kV, 33 kV, 66 kV, 132 kV, etc.
The reason that 110 V service was chosen, was economic. Originally electricity was delivered to homes, and most businesses, for a single purpose and that was lighting.
Edison selected 110V because it provided the best compromise between distribution costs and lamp efficiency for carbon bulbs. So those were the systems deployed in the United States. Later, when higher-voltage metal filament bulbs were developed, 220 volts was the optimal voltage. 220 volts became popular for newer systems first in Germany and later in all of Europe and in our country INDIA also.
But basically, it's fairly arbitrary.
The voltage and frequency of AC electricity vary from country to country throughout the world. Most use 230V and 50Hz. About $20\%$ of the countries use 110V and/or 60Hz to power their homes. 240V and 60Hz are the most efficient values, but only a few countries use that combination.