It is hard to calculate since there are several factors that are needed and which you need to consider. First is to  gather all the bitcoin volume for sale on all exchangers and get all the value attached to it. After that you compute the number of bitcoins volume to their corresponding price to get the amount equivalent per volume. Then as you go computing the volume together with its equivalent price then you will know how much bitcoin you will get by using 10 billion USD. But take note that traders are smart so they will remove their sell order and set it to a much higher sell order.