Almost every written source for Mexica religion postdates the destruction of the society that practised it, and the few that do not are hard to read.
Pre-conquest books were painted screenfolds of bark paper or deer hide. Very few survive from central Mexico, and the origin of the best-known group is contested. The Mexica books themselves are gone, destroyed in the conquest and, according to a later Nahuatl account, already purged once on the orders of the ruler Itzcoatl so that an approved version of the past could stand. What survives is largely from neighbouring peoples, and is pictorial rather than a script recording speech, so it can be interpreted only with help from later commentary.
Colonial compilations supply most of the detail. The largest is the encyclopaedic work assembled over decades by the Franciscan Bernardino de Sahagun, working with Nahua assistants and elderly informants, written in Nahuatl with a parallel Spanish text that does not always say the same thing. Diego Duran, Motolinia and others wrote comparable accounts. These are indispensable and compromised in the same breath: gathered by or for men committed to eradicating what they described, from informants with their own reasons to shape the record, and sorted into European categories of god, idol and devil that may not fit what they were sorting.
Archaeology is the independent check, and has grown far more important since the accidental discovery of a large carved monolith in Mexico City in 1978 led to the systematic excavation of the main temple precinct. Offerings sealed inside successive construction stages can be dated and could not have been edited afterwards. They have confirmed some colonial descriptions and complicated others.
The defensible position is that Mexica ritual is known in considerable detail, that its logic is known in outline, and that statements about numbers, or about what individual people believed, deserve far more caution than they usually receive.