1) If the letter X comes at the end of the word, it is pronounced as क्स (ks)
Tax टैक्स्
Six सिक्स्
2) If the letter X comes in the middle of the word and letter X is followed by a consonant (sound), then also it is pronounced as क्स (ks)
Excel इक्ˈसेल्
Expect इक्ˈस्पेक्ट्
3) If letter X is followed by a short vowel sound, that time also it is pronounced as क्स (ks)
Taxi ˈटैक्सि