The way you frame your questions makes a misleading assumption, that the “Aztecs” where a genetically isolated group (kind of like a biological specie). The society that it is commonly referred as “Aztecs” was a conglomerate of different identities and polities. It was a true cosmopolitan society that was, one way or another, interrelated with the plethora of different societies that inhabited the cultural area of Mesoamerica during the Postclassic period. For example, during the same time period you have the Zapotecs, Mixtecs, Chatinos, Triquis, Chinantecs, Tacuates, Mazatecs, Chochos, Amuzgos, Cuicatec, Mixes, Zoques, Huaves, Chontal, just to name a few of the indigenous communities from what is now the modern State of Oaxaca. So to say that modern Mexican indigenous population are only the Nahua communities from central and Southern Mexico is a gross misunderstanding of the cultural complexity that it was found at the time of the conquest, and that is found today.

Now in regard to the essence of your question (how many indigenous people are living in modern Mexico), according to CELADE, there are about 17 million indigenas in modern Mexico. However, there are various problems with this number. For example, Mexico traditionally defined indigenas as any person who spoke an indigenous language from Mesoamerica. Defining indigenismo in this way had the tendency to gloss over indigenous communities that for historical reasons had lost their mother languages in favor of Spanish. It also did not count indigenous urban communities, and favored the rural ones. Furthermore, there were communities that, again for historical reasons, were so isolated that pretty much felt out of the radar. So defining indigenismo under linguistic terms is simply a flaw methodology.

Mexico praise himself for being a country of Mestizos, but again what the hell is this mean. The term mestizo in the Mexican context is a very political one that sprung out of the 1910-1921 revolution that created the modern Mexican identity. In reality, according to Juan Pedro Viquiera, we really do not know what happened during the 19th century. By the beginning of the independent movement (1810), around 60% of the population was classified under the juridical label of “indio”. I should emphasize that the casta system during colonial New Spain was not based on ethnic, biological, or racial terms, but on juridical ones; in other words, to be classified as an “indio” did not meant that you were of “pure indian” descent (whatever the hell purity may mean), but that you had to pay taxes as indio. When we again get reliable census data, in 1900, turns out that there are only 15% of people under the label indigenous (the census used language as a definition for indigenous). However, the problem is that by now, there are racial terminologies that obscure rather that shed light on the process of mestizaje.

According to Mauricio Tenorio Trillo, after the revolution, the paternalist state creates the notion of mestizaje to advance the agenda that everybody is the same, a mestizo. However, mestizo now becomes a synonym for a benefactor state. If to be mestizo is to have access to state institutions that brings everyone under the umbrella of a dictatorial and corporativist government, but that a the same time legitimizes such corrupt government, then everyone was a mestizo. However, this includes the elimination of “the indio”, the elimination of afro-descendant, elimination of Chinese descendant, or of any “minority”, and the homogenization of the Mexican population.

I think I went a little beyond your question, but it is time to stop thinking in “purity” terms that do not reflect a biological reality.

In Mexico Indigenous identity is usually determined by the language you speak. So you might be of Nahua descent but if you don't speak Nahuatl you probably wouldn't be counted. According to Ethnologue, there's 1,380,000 Nahuatl speakers today.

The CIA estimated that 30% of Mexico's population is a majority American Indians, while 60% are mestizo.