I have a reservation for a patio table at 8:00 pm.
I have reservations about recommending you for the job.
Now, I suspect you're probably talking about something similar to my first example. A restaurant, a hotel, or a spa perhaps? But since I have no context to base my answer on, I can't say for certain. If my hunch is correct, then the two can be used interchangeably most of the time. The only time I would definitely say "reservations" is if we were a party of 30 checking into a hotel, or a flight for an airline or a restaurant. But that is just me - there is really no obligation. You, as the speaker of the couple or the group, could use either.