With the exception of some foreign adjectives, adjectives in the plural uniformly end in -и. If an adjective ends in a consonant in the singular, the plural ending -и is added. If an adjective ends in a vowel in the singular, the vowel is replaced with -и in the plural.
As you may remember, some adjectives undergo transformations from the masculine to the feminine and neuter form. The same transformations occur in the plural. One of these transformations is a vowel in the penultimate position (next to last in the word) that exists in the masculine but disappears in the feminine, neuter, and plural forms. For example, тесен, тесна, тесно, тесни 'narrow, tight'. Another transformation occurs to the consonant before the disappearing (fleeting) vowel. For example, жежок, жешка, жешко, жешки 'hot.'