The number of tubes allowed to be plugged is generally determined by the pressure drop and heat transfer characteristics of the exchanger. One must complete a sensitivity analysis of the unit with respect to number of tubes. For instance, an exchanger that is heated by steam on the shell side that is not normally operated at the full steam header pressure may have a great deal of flexibility in plugging tubes, perhaps even up to the pressure drop constraints on the tube side. There can be cases, however, where even 10 percent of tubes being plugged could have unacceptable process performance. Mechanical issues also exist in the plugging of tubes, although I cannot comment on these as it is outside my knowledge base.