How much physical activity do babies need?

As much as they want. Babies are great at deciding how much exercise they need! keep them safe and let them roam. You will see cues when they are tired.
Physical activity. It depends on the age of your baby. Here's a great resource regarding physical activity at different ages: http://prod.Healthychildren.Org/english/healthy-living/growing-healthy/pages/baby-activity.Aspx.
It varies by age. Babies pretty much 'know' how much exercise, sleep, & food they need. Watch for their cues. Of course, they need lots of love, cuddling, positive words and interactions, ( & clean diapers.