Magento: Set product quantity default to (1)

Step 1. Go to Admin Panel -> System -> Configuration -> Inventory-> Product Stock Options
Step 2. Now Click on Add Minimum Qty button
Step 3. If you want to apply it in all groups, choose Customer Group: ALL GROUPS
Step 4. Enter 1 for Minimum Qty
Step 5. Then Save.

Clear your cache and try to navigate in one of your product page.