Can a metallic sphere of radius 1 cm hold a charge of 1 coulomb?

Well I doubt it would be able to hold a charge of that density in air. When a sphere of radius 1cm is charged to such a high level, the electric field will be far larger than the dielectric strength of air. Hence, a breakdown voltage will be achieved and electrons will flow even through air. It's the same principle that lightning works on. 1 coulomb is a huge amount of charge, and the surface charge density in this case will be too high for air to take. So, a sphere of radius 1 cm cannot hold 1 Coulomb of charge.