Make the reservations for seven people. It's much easier to change to a smalller party than to a larger party. As soon as you know if the one person cannot make it, call to see if there would be a problem to drop the one person off. If there is (you might have to switch to a slightly different eating time), then keep the original reservation and check at the podium of the restaurant that day and tell them of the change in party size. Otherwise, you can show up with just six. In other words, do everything you can to notify the restaurant of the change. It's kind of like when you are planning a dinner party and someone doesn't show up for it. It could or could not be a problem for you.