How do you say 1989100000000000000000?

To say the number 1,989,100,000,000,000,000,000, you would start by reading the whole number before the decimal point as "one sextillion, nine hundred eighty-nine quintillion, one hundred quadrillion." Then, you would read the decimal point as "point" and continue with the numbers after the decimal point individually; in this case, there are no numbers after the decimal point, so you would simply end at "quadrillion."