Hebrew numbers come in two forms: masculine and feminine. The form depends on the gender of the noun being counted. The feminine form is the default for abstract counting (phone numbers, time, prices). The masculine form is used for masculine objects (books, men, days...).
Default counting, When counting without a specific object (1, 2, 3, 4...), use the feminine form: achat, shtayim, shalosh, arba...