It would generally be best to have one rose in each container although if the containers are very big they should be Ok. Have the buckets got drainage holes in the bottom? If not the lack of drainage will cause the roots to rot.

Have any of the roses developed new growth? If so but others haven't it may be the ones that haven't have died although I'd give them a few weeks to be sure.

Roses should develop new buds in early spring and get new leaves a little later.

Just how big are your 'big buckets' Unless they are extremely large then having more than one rose bush in them is pushing it. And, as Anchorman says, there needs to be drainage holes in the bottom which there normally isn't with a bucket - sort of defeats its purpose!