Using some code Shutty found on Yahoo I was able to see the impact of limiting the maximum number of trades per week. I added the code in did an optimise and exported the results to Excel to get the graph above.
For a longer term system, like the one used for this test, it is possible to limit the number of buys to one per week and still get reasonable results.
With a shorter term system limiting the number of trades per week would have a bigger impact since the system needs to trade more - see the results for a 4 week system below.