It's the nature of waffles to get soggy as they sit, as the steam inside escapes through the outer crispy coat. It's really best to eat them immediately, but I understand how you want to hold them until you have enough to serve everyone at the same time.
When storing them for a short time in a 170-200° oven, put them directly on the oven rack and don't stack them on top of each other. You should be able to hold them successfully for about 20 minutes this way.
Hope that helps.