How much blockage must there be of coronary arteries to be recommended for bypass surgery versus just getting stent?

Not about degree : it is not about the degree or percentage of blockage. It is about the location or number of areas that have disease. There are also other factors that play a role such as history of diabetes or poor heart function. Most commonly: if the left main artery or all three of the major coronary artery's have significant blockage then surgery is recommended.