Check out this site for clarity, but we usually say "Gojyu- on" which is 五十音 as traditionally, we used to have fifty one ひらがな and another fifty one カタカナ in Japanese, but 5 out of them are not in use expect in reading classics so that we have now 46.　 Hiragana letters were created from Chinese characters in the 9th by noble women who loved to read and write poems and journals while katakana letters were made using a part of Chinese characters, to keep official records and read Chinese literature easily, slightly earlier than when Hiragana was created.

A nitpick: it's a syllabary, not an alphabet. The magic number cited is 50 (5 vowels x 10 initial consonants), but there are holes in the table. It also depends on how you want to count diacritics and digraphs. Have a count for yourself at: http://en.wikipedia.org/wiki/Kana