Hello, it's impossible to tell how many eggs your turtle will lay. She may lay them all at once or spread them out into a few sessions... one turtle on here has been gravid four times this summer... it's really an individual trait.
As for the nesting site, here's a link on nesting info:
http://www.redearslider.com/reproduction.html
Also, here's a link on creating "natural-esque" conditions for gravid turtles:
http://www.tortoisetrust.org/articles/Nestsites.htm
Place her in the nesting site anytime she seems antsy and is trying to escape the tank- that's a good sign that she wants to get out to lay eggs. You can leave her out a few hours, it may take her a while to feel safe and comfortable.
Finally, here's a link to one of the other topics about gravid females:
http://www.redearslider.com/forum/viewt ... ht=#108509
Good Luck!