Can I reuse an alginate bath for reverse spherification?
This site say you can use it again, just make sure you strain it before you store it
http://www.molecularmixology.co.uk/index.php?act=viewRecipe&amp;recipeId=34

This site says you can store it for a couple of days in the fridge
http://www.molecularrecipes.com/spherification/7-tips-making-spherification-caviar/

As for how many spheres can be produced - I'm afraid that would be completely dependent on how large you make your spheres.