Counting rounds doesn't work well for me, I start having fun and lose my count. But try this: Load the first round in the magazine with a quiet LR round like the Aquila Super Calibri or CCI Quiet. The difference is sound and feel will tell you it's the last round shot. By the way, it may not cycle the slide, but it does keep you from an unnecessary dry fire.
Shoot often, shoot safe and have fun.