1. A grain of corn is on the table.
2. Two grains of corn are on the table.
(Are they OK? Please focus on the plural forms?)
3. The eighth square on the second row needed 256 spoons of rice, which amounted to a bowl.
(In this sentence 'which' refers to '256 spoons of rice, so 'which' can be changed into, 'and they' or 'and it'?)