It depends if they Bears wouldn't be scared of the T-Rex,
If we are saying that Bears WOULD attack the T-Rex regardless of the fact it's way bigger, more powerful and no doubt a large portion of the pack would die, then I reckon about 25-30 bears would do it. 50% of them would get Nommed, but the T-Rex would fall eventually.
I just don't think the Bears are nimble enough to avoid it's wrath. Thinking back to the Raptors in Jurrasic Park, they were so fast and had huge tallons to dig in and hold onto the animal.
I would probably think 20 - 35 lepoards might do it as they are fast enough to avoid being eaten.
maybe only 10 Comodo dragons as they have poisonous bites? the T-Rex would die eventually.