i'd like to see the source of such statement. there were human sacrifices but modern historians think they happened in small numbers. the "thousands" version come from the spanish conquerors, that used this as an excuse to destroy those ancient civilizations saying they were barbs... it is a good question about civilization, who was more barbarian? those that made those crazy sacrifices cos their religion, or those that destroyed thousands of years of culture?
Modern historians generally agree that the Aztecs did, in fact, sacrifice thousands.
How many thousands is a bit contentious, but certainly still thousands. In the pre-Columbian Ramirez Codex, the Aztecs themselves (before they ever met a Spaniard) claim to have sacrificed 80400 in 4 days to consecrate the Templo Mayor when it was first raised. This is surely exaggeration, and most historians go a tenth of that figure or less because the rate exceeds that of modern capabilities in slaughterhouses and death camps and simply wasn't physically possible given the facilities at the Templo Mayor. According to Aztecs interviewed by Spanish missionaries after the Conquest the number was 4000 in four days, far more plausible.
You must be thinking of the Maya, or perhaps one of the other Mesoamerican groups (Mixtec, Zapotec, etc). These groups also practiced sacrifice but in far, far more limited numbers. The Aztecs were an exceptional group, who were not native to the area but had arrived from the north relatively recently.
Of course it all has to be taken in context. By and large, these were not civilians rounded up and forced under the priest's knife. Generally, these were prisoners gathered in the Flower Wars, which were sort of like pseudo-warfare with very low casualties. In European wars, these individuals would have been the wounded and maimed left on the battlefield, who were generally put under a knife by the winner there as well, just in a less ritualized fashion (hospitality towards prisoners was accorded only to nobility, until fairly recently). If I came afoul of some historical state, I would rather be quickly killed in a surgical manner by an Aztec priest, than undergo an Elizabethan execution: hanging til I nearly lose consciousness, then having my guts slowly torn out by a winch, living just long enough to watch them feed my innards to dogs or pigs.
For that matter, Europeans also practiced ritual human sacrifice to please their deity at the same time. What else could one call the execution of heretics and witches?