How many volcanoes can erupt at once?
In theory, there is no limit on the number of volcanoes that could erupt at once other than the number of active volcanoes themselves: while it is thus theoretically thinkable, that all 600 volcanoes (on land) known to have had eruptions during recorded history erupt at once, this is so unlikely that it can be excluded.
At any given time, about 10-20 volcanoes are erupting on average, and it could be imagined that this number sometimes peaks to about 30-50 erupting volcanoes (on land). Many more than that is very unlikely. Including submarine volcanoes, these figure probably have to be multiplied by a factor of at least two or three, but much less is know about the activity of submarine volcanoes.
In Earth's earlier history, there have been times when there were many more volcanoes active than today. Unless the Earth's present geodynamic system changes dramatically, of which there is no indication of happening in any foreseeable future, such levels of volcanic activity will never return.