one syllable, ai is one vowel, sounds like the ay in play

No, there's only one syllable in rain. a and i act as one vowel sound, and the r and n are connected. Pronounce it while clapping, and you'll notice there is only one beat. rain.

It has just one syllable: The vowels 'a' and 'i' ombine to form one sound. The 'ai' makes a single sound as in 'day'.

One long-vowel syllable, as how Veronique describes it.

only one syllable . [rein]