how many syllables are there in once?
Word Count 1
Syllable Count 1
Character Count (alphanumeric) 4
Mean syllables per word 1.00
Mean characters per word 4.00
Yes. Air and they're rhyme.
There's only one syllable in once.
how about in first and they're and week and field?
I suggest you look these words up in a dictionary to find the number of syllables in each.
Here's a good online dictionary.
http://dictionary.reference.com/