Horse riding involves a great deal of physical effort. The up and down movement can easily start bleeding. You should not ride for at least two weeks. At two weeks walking is usually possible but you should not trot or canter for four weeks. Once the surgery has completely healed there is no restriction on any kind of riding, jumping or eventing.
Following a breast augmentation you should wear a good supportive bra whenever you ride. For the first month or so you may feel more comfortable using an Ace wrap on top of the bra, or instead of it.