Before Thomas Aquinas denounced suicide as a crime punishable by both law and excommunication, were the numbers of suicide increasing since the spread of Christianity?

We have nothing resembling suicide statistics for the Middle Ages and late antiquity. Absolutely nothing. In his epic (truly) study Suicide in the Middle Ages, Alexander Murray talks about how elusive even references to suicides are--"whispers," he calls them--a result of the terrifying consequences medieval Christians believed suicide to call down.

But getting away from statistics for a spell, I think you might be working under a misconception. Thomas Aquinas didn't invent punishments for suicide in this life or the next. He's massively important in medieval and Catholic theology, but on this particular issue, not that innovative.