Two. I have a heavy U lock for any time the bike is going to be out of sight. I have a heavy cable combination lock for any time I am dashing in and out and probably have the bike in sight the whole time.

I use both locks if I have to leave the bike for a long time. Both locks may not stop a determined thief... but most likely they will look elsewhere.

I also ride a bike that looks like it has seen better days... that alone probably helps. Back for a year in my college days I rode a rusty stepthru frame bike with rusty fenders and a bent up basket on the handle bars... parked it on campus just about anywhere, with no lock. No one ever touched it. Sold it at the end of that year for a 6 pack. Internally the bearings and moving parts were in great shape, externally, it looked like it was all rusted together.

I don't leave my sleek shiny red bike anywhere. It attracts attention.