Q.When should I grow hollyhocks from seeds
I want to grow hollyhocks in pots on my terrace garden. Can someone guide me as to what whether I should grow them from seeds? Also, how deep should the pot be so it has enough soil to grow?

Yes, you can grow Hollyhock in containers, but you will need quite large pots to grow this tall flowering biennial.
They can grow 5 to 6 feet tall. A good container would be a half whiskey barrel or something in that size range.
Here is a link with growing information.