Thanks for asking this very important question. In life, many things are minor but have great impact in future. Cover blocks is one of the very minor but important item in civil engineering concrete work of slabs, columns, beams etc.
There is no rule for how many number in how much area. It is on site decision as per requirement and as deemed fit by the executing body. But when you have raised the question, let me attempt.
Say for a room roof slab of 120 after area, skilled labour one number with two numbers helper is doing work. So, he may use a wooden plank for sitting and doing work. So, in 4 sqm area, at least we will give 4 blocks. Adjoing to it only 2 new will require. So in 8 sqm, we need 6 blocks. So in 80 sqm, 60 blocks. Likewise for 120 sqm, 100 cover blocks required.
Now it looks too much. Some may object. Then what to do? So, to protect the position of steel bars, we have to use the plank of big size, then instead of 100, we may use 60 to 70 cover blocks. For giving concrete by helper, they should not walk over the laid reinforcement. They should walk on a plank and pour the concrete. Now pouring the concrete even disturb the position of 8 mm bars. So we have to educate the working labours about importance of placing cover bars. Contractors don’t understand and use very less blocks resulting bad quality of RCC work.
Aim it to protect the position of steel and allow concrete to go beyond steel to provide a cover and protect steel from weathering, rusting and develop proper strength.
Same for column. In column, number of block is less as it is vertical and once shuttering is done properly, for 3 meter length, on each face 6 to 8 cover blocks is sufficient.
Hope it is clear now. Take your own decision. No hard and fast rule. Thanks again