Well width and depth play an important part as well, but in a tank that size you could fit quite a few! I had a breeding group of 7 in a tank ~1200 gallons, and they did quite fine in there.
In a tank with 35 feet of straight travel, assuming its at least 6-7 feet wide, you could house 30+ adult rays comfortably.
Also @FelipeDBKO, a fish keeping form will be nigh on useless for this question. Very, very, VERY few people keep pelagic ray species privately, and of those who do, they often only house 1 or 2 juvenile specimens in systems that would not be able to handle groups of adults, so are perhaps not the best sources of advice for stocking densities.