Is it true that uttering "Astaghfirullah al-Azim" (I seek forgiveness from Allah, the Almighty) one hundred times is counted by Allah as one hundred times?

What you heard, that the reward for seeking forgiveness is not multiplied, is incorrect. Rather, the reward for an act of obedience is multiplied tenfold, and it may increase to seven hundredfold, or even more. As stated in the Hadith Qudsi: "Whoever comes with a good deed, he will have ten times its like, or more." This is what Al-Nawawi and Al-Iraqi held: that the multiplication does not stop at seven hundred, but can increase to many, many folds.