Oh yeah a pen is good, too. We have a pen for our ducks ( we have 10!) and we put them in it at night and we are grown and when we're home we let them walk around in the yard.
A lot of people have only 1 duck. It's ok. As long as you give her plenty of attention she should be alright. I just know how much my ducks enjoy one another's company.
She might even be afraid of a new duck at first since all she knows is 'you'. She'll be fine without another duck friend. But, (if your like me) you probably have a lot of other things you have to do and can't be with her all the time. I mean I started out with 1 duck and felt sorry for her, because I wasn't with her all the time ( and you know how their little faces look when they look upset or something) so I went and got her sister..and befoe you know it 3 years later I've got 10 ducks!
I know how messy they can be. When they're younger they're more messier and hard to take care of. It's much more enjoyable when they get big enough to put outside. Then you can walk out and enjoy them when you want to.
Did you name her? 
but yeah I think if 1 is all you want to get and you have plenty of spare time to give her then that's fine to only have one. The main reason you would want to get another is if you can;t give her much attention and you sound as if you really love her and play with her a whole lot!
Genny