how can i investigate the effect of starch concentration on a fixed amylase concentration?

First of all, I don't know what you mean by a concentration of 1C.  Concentrations are usually given in terms of molarity or percent by weight.  If you mean molarity, it is unlikely you could get something like 5 M into solution.  Why not do a series of dilutions by half... start with a 20 g per liter solution"Prepare a 20 g/l starch solution.
Mix 20 g of soluble potato starch in approx. 50 ml of cold water.
While stirring, add the slurry to approx. 900 ml of gently boiling water in a large beaker.
Mix well and cool the gelatinized starch solution to room temperature.
Add more water to bring the total volume to 1 liter.
Put a few drops of the starch solution on a glass plate. Add 1 drop of the iodine reagent and see that a deep blue color is developed. The blue color indicates the presence of starch in the solution."

Then take 100 ml of this solution, and add 100 ml of water, and you would have a 10 g/l solution, and so forth...

Once you have your starch solutions you could put them in test tubes.  At a given time, add the same amount of enzyme to each, and shake them up.  Have pipettes and spot plates ready, and take a couple of drops from each tube every minute, put it on the spot plate, and add the iodine solution (which would stop the reaction).  

You could graph the amount ofl the starch is broken down against time.