Only those adjectives which end in ा in their standard (masculine singular) form change to ी for a feminine subject and े for a masculine plural subject. Eg: बड़ा(big), थोड़ा (some) , काला (black) etc.
Other adjectives do not change according to gender and number. Eg: जवान (youthful), लाल (red), दुखी (sad) etc.
There are some exceptions to the rule though. Eg: ज़िंदा (alive) which doesn't change form.