Which suffix should be used to pluralize the following word ? taxi

Concept: English regular nouns form their plural by adding "-s". Certain endings require a different suffix: nouns ending in a sibilant sound (s, x, z, ch, sh) take "-es" (bus → buses, box → boxes); and a small closed set of irregular nouns take "-en" or "-ren" (ox → oxen, child → children).

Application: "taxi" ends in the plain vowel sound "i", not in a sibilant sound, and it is not one of the irregular "-en"/"-ren" nouns, so neither special rule applies — the regular rule governs, and adding "-s" gives "taxis".

Cross-check: "-es" would apply only if "taxi" ended in s/x/z/ch/sh (it doesn't); "-en"/"-ren" apply only to the fixed irregular list (ox, child, and a few similar words), which "taxi" is not part of. A rare, non-standard spelling "taxies" occasionally appears, but mainstream dictionaries (e.g. Collins, Britannica) recognize only "taxis" as the standard plural. So "-s" is the suffix consistent with how "taxi" ends and the one used in standard English.