What leg exercises are recommended if I only have one leg?

If you only have one leg, then perform single-leg exercises. Start by practicing balancing on one leg (without weight) by slightly bending the knee, drawing-in the abdominals, and contracting the buttocks.  Once balancing becomes easier, you can perform exercises such as single-leg toe touches, single-leg squats, and eventually, upper body exercises, such as biceps curls, while balancing on one leg.