They do say at least one. However, if you do only one, they wont penalise you. They say at least one, so clearly a minimum of one. I prefer to do only one and go into a greater depth than two and only talk about them generally.
Our trial exam stated at least one, and I only did one and I managed to get 14/15. So I doubt they'd penalise you for only doing one. Remember, you only have 40 minutes- if you do 4 texts overall, that's less than 10 minutes for each text (because of the introduction and conclusion).