Question: how to calculate how much virus to add?
Answer: MOI is related to pfu by the following formula: Multiplicity of infection (moi) = Plaque forming units (pfu) of virus used for infection / number of cells. For example, if 2x106 cells is infected by 50 ml of virus with a titer of 108 pfu/ml. The moi will be 0.05*108/2*106 = 2.5.