Hello Sally,
And the one in your question,which contains 21 zeros :
1,000,000,000,000,000,000,000 = sextillion
If you are interested, the following each number with 3 more zeros after the 1.
1,000,000,000,000,000,000,000,000 = septillion
1,000,000,000,000,000,000,000,000,000 = octillion
1,000,000,000,000,000,000,000,000,000,000= nonillion 1,000,000,000,000,000,000,000,000,000,000,000= decillion