How much topsoil

It depends how deep a layer you want. For every cubic metre of topsoil you buy it will give you about 8mm depth approx spread over 120 sq m. A cubic metre is about 0.7 tonne depending on the type of topsoil and water content and gets delivered in a big builders bag. It's going to be expensive I think to cover that area with say 4 inches (100mm) - you would need about 13 cubic metres. If you have good access it would be cheaper to buy 2 lorry loads and dump it loose. It depends on your budget but I would recommend trying to level the soil you have and remove most of the bigger bits of rubble.

What depth of soil have you already?

You will need to compact it well, a roller would be good, maybe you could hire one or a cheaper method is to tread it down using your heels. It's a bit time consuming and means walking on your heels over every bit of ground then filling any depressions then compacting again. It does work provided you have only disturbed the top layer of soil. After compacting use a rake to create a nice seed bed if you intend to sow it yourself. It will need plenty of watering especially on sunny days.