There will be more than the one egg but it's hard to say how many. Some turtles on here have laid more than a dozen in a laying session. I've heard they can even lay up to twenty! but that would be unusual. I think sometimes it's only four or so eggs. All over the map.
It's not a good idea for her to lay in the water. They don't like to do that and will be inclined to retain their eggs rather than lay them in the water. She wants to get out to a place where she can dig in the dirt and then bury the eggs.
She needs a nesting box. There's a description in another recent post - I'll go try to find it.
EDIT: If you keep a separate setup for a nesting area, then you do not need to have water in there. It should have a 50/50 mixture of sand and organic compost. It should have a light, be as large as possible and have some privacy. It should only be necessary to put her in there when she is showing signs of wanting to lay eggs (restless and digging behavior).
Please read these pages:
www.tortoisetrust.org/articles/Nestsites.htm
www.redearslider.com/reproduction.html
That was a post from Steve, the moderator on here.
The turtle will need hours in the box. If she doesn't lay at first, try her again the next day. If she refuses to lay in a good nesting box, come back and let us know.
She probably won't lay all of her eggs at one time, she'll need several sessions. You'll know when she's through laying because she'll calm down. They she'll need some good food and lots of calcium to replenish what she's lost.
