First of all, Mesoamerican civilisations were not tribal. They were city-states that formed confederations with each other, some going further and becoming Empires. This is not a mere technical point, and the distinction is important for your question. As city-states, Mesoamerican civilisations had complex hierarchies, structured institutions, and a highly developed system of beliefs which tribes typically lack. These features are essential for understanding human sacrifice in Mesoamerica. Institutions offered incentives to both perpetrators and victims of sacrifice, who were rewarded with status within the hierarchy (though victims usually didn’t have too long to enjoy it). This system was justified by the Mesoamerican religious beliefs which states that the earth was controlled by a series of unstable supernatural cycles. Religious worship, of which human sacrifice was only a small part, could help to re-order, even reset, those cycles, ensuring that the earth would remain fertile and stable, and disasters like drought could be avoided. This may explain why there wasn’t much opposition from within Mesoamerican societies. The only evidence comes from the accounts of Fernando de Alva Ixtlilxochitl, who claims that his ancestor Nezahualcoyotl opposed sacrifice and preferred the worship of a peaceful, invisible god. However, his account is very suspect, and has been deconstructed by Jongsoo Lee, who notes that the real Nezahualcoyotl was an enthusiastic follower of traditional Mesoamerican religion, and the ‘peaceful, invisible god’ never existed. Most likely, Ixtlilxochitl was trying to recast his ancestor as a proto-Christian, in an attempt to gain concessions from the Spanish Crown. That said, some (unspecified) individuals were reported to have panicked in the moments before sacrifice. But this seems to have been a rare event, rare enough at least to be seen as a bad omen.
As to the first question, frankly we don’t know the answer. Almost no ethnographic evidence exists for most Mesoamerican civilisations. Spanish accounts focus heavily on the Mexica, and this can be a little distorting (for reasons I will explain in a moment). Furthermore, they also give confusing, contradictory, and overly inflated figures for victim numbers. So far, archaeology has not supported their numbers. Returning to the Mexica, a reading of Sahagún and Durán suggests that the Mexica would typically sacrifice about 300 or so people a year. Occasionally they would have a large sacrifice, such as after a major war. These are the events where Spanish sources typically give large numbers, but we do not really know how big or frequent they really were. They may have had a few hundred victims each.
We must be careful about extrapolating this number to other Mesoamerican nations. The Mexica probably sacrificed a disproportionately large number of people than other civilisations, for two main reasons. First, the Mexica were leaders of a military empire, and so could take the jaguar’s share of captives from their allies. Second, they absorbed the rituals of peoples they encountered. Thus, the Mexica not only had larger ceremonies, they also had more of them. Furthermore, when I say absorbed, in some cases that was literal. Many of the rituals conducted in the mountains surrounding the Valley of Mexico were attended by leading members of all cities within the Basin. Thus, these ceremonies were consolidated into the single ritual scheme controlled by the Mexica. This may have actually lowered the overall number of sacrifices conducted within the Valley. Thirdly, not every ritual may have had a sacrifice every year. In other words, they may have alternated. This may explain some of the inconsistencies in Spanish accounts. For example, in The Book of Gods and Rites and the Ancient Calendar, Durán claims that the Quecholli festival had no human victims, but then contradicts himself in another part of the same volume. If correct, then this may indicate that the number of sacrificial victims was even lower than the total.
To conclude, we don’t really know how many people were sacrificed or how frequently they were sacrificed. However, in most places it was probably infrequent and in small number, at least relative to the overall population. The Mexica practised it more than most and more frequently. Even then, sacrifices were usually fairly small, unless a big war had just been fought.
Sources: Benson, Elizabeth and Boone, Elizabeth H.: - Ritual Human Sacrifice in Mesoamerica, (Washington D.C.: Dumbarton Oaks, 1984)
Carrasco, Davíd: - City of Sacrifice: The Aztec Empire and the Role of Violence in Civilisation, (Boston: Beacon Press, 1999)
Durán, Diego: - Book of the Gods and Rites and The Ancient Calendar, tr. Fernando Horcasitas, and Doris Heyden, (Norman: University of Oklahoma Press, 1970) - History of the Indies of New Spain, tr. Doris Heyden, (Norman: University of Oklahoma Press, 1994)
Keen, Benjamin: - The Aztec Image in Western Thought, (New Brunswick, New Jersey: Rutgers University Press, 1990)
Kerkhove, Ray: - ‘Dark Religion? Aztec Perspectives on Human Sacrifice’, Sydney Studies in Religion (2008)
Lee, Jongsoo: - The Allure of Nezahualcoyotl: Pre-Hispanic History, Religion, and Nahua Poetics, (Albuquerque: University of New Mexico Press, 2008)
López Luján, Leonardo: - The Offerings of the Templo Mayor of Tenochtitlan, tr. Bernard R. Ortiz de Montellano and Themla Ortiz de Montellano, (Niwot: University Press of Colorado, 1994)
Motolinía, Torbio de Benevente:
Nagao, Debra: - Mexica Buried Offerings: A Historical and Contextual Analysis, (Oxford: BAR International Series 235, 1985)
Ortiz de Montellano, Bernard R.: - 'Cannibalism: An Ecological Necessity?', Science New Series 200/4342 (1978)
Pennock, Caroline Dodds: - Bonds of Blood: Gender, Lifecycle and Sacrifice in Aztec Culture, (Hampshire: Palgrave Macmillan, 2011) - ‘Mass Murder or Religious Homicide? Rethinking Human Sacrifice and Interpersonal Violence in Aztec Society’, Historical Social Research/Historische Sozalforschung, 37:3, (2012)
Restall, Matthew:
Sahagún, Bernardino de: - General History of the things of New Spain Book 2: The Ceremonies, tr. by Arthur O.J. Anderson O.J., and Charles E. Dibble, (Santa Fe: University of Utah, 1979)
Sigal, Pete: - The Flower and the Scorpion: Sexuality and Ritual in Early Nahua Culture, (Durham: Duke University Press, 2011)
Smith. Michael E.: