Well, if the OP uses the 35mm then f4 should be about right as that should give plenty of DoF and will hopefully blur out the background but there are other factors. The distance from the subjects to the background for instance. If the OP wants to use the 18-55mm then, assuming he isn't using the 18mm end, f4 will unlikely be an option.
Either way, stating an f number as fact is probably less than helpful.
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