It's usually takes about 10 to 30 tranq darts to knock out a rex so you shouldn't use tranq Arrows unless it low level and desperate. And it also takes 10 to 100 narcotics to keep it asleep until you tame it

The easiest thing to use to tame it is prime fish šŸ–.

So get a shark first and finally the best tip of always over prepare and use more tranq darts than necessary.

The best place to find a Rex is on the volcano in the middle of the map.

More Rex Taming & KO Tips