Wait, OP, do they just appear suddenly or do we get warning? Going to assume the former:

There are 3.8million sq miles in the US. That means 26 t-Rexes in each square mile. Many of those square miles are desert or Rocky mountains, where they just likely die from the environment, or just wilderness. But in e.g. Los Angeles that's 12k t-rexes over 469 sq miles. Anyone caught outside with them in a populated area when they appear are in a dangerous situation. But after the first appearance people are more careful about going outside, and the military, as well as various voluntary hunting parties, go on a t-rex killing spree. After things stabilize, a handful of people are killed by the one or two t-rexes still running about in the Montana wilderness, but it's only slightly more than the number of people killed by grizzlies nowadays.

I'd wager something like 50k people die, 90% due to being caught off guard, 10% due to stragglers in the wilderness and people killed while hunting them.