I have analysed the human gut 16s data with (qiime2-2021.11) and detected less number of total species than genus (139 vs 252) in the overall data. Is this normal?
Taxonomic classification was conducted with gg_13_8 database.
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With 16S data, species-level annotation is not very reliable due to relatively low resolution. Therefore, many sequences fail to be annotated to the species level and annotated only to Genus. In that case, you have more genera than species.
BTW, with 16S amplicon data it is recommended to work at the genus level regarding taxonomy.