A Googolplexianth has an incomprehensibly large number of zeros.
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A Googolplexianth is a number so large that it cannot be written out in standard form or even imagined in its entirety. It is the digit 1 followed by a Googolplex number of zeros, which is itself a 1 followed by a Googol zeros (a Googol is 10^100).
To put this vast number into perspective, astronomer Carl Sagan once stated, “There are more stars in the Milky Way galaxy than there are atoms in the universe. We know this because each star is essentially a giant fusion reactor, and we know how many atoms of hydrogen it takes to make fusion happen. We can also calculate how many atoms of hydrogen there are in the Milky Way. This number actually has a special name, it’s called a googol….So, mathematically, it’s possible to design a number bigger than the number of atoms in the universe. It’s not just improbable, it’s impossible to write it down in any meaningful way. This number is called a googolplex.”
A Googolplexianth takes this incomprehensible number even further. Trying to visualize a Googolplexianth is like trying to imagine an infinite universe or the concept of eternity.
Some interesting facts related to this topic include:

The term “googol” was coined by the mathematician Edward Kasner’s nineyearold nephew in 1938.

If you were to write out a Googolplex in standard form, it would be a number with more digits than there are atoms in the observable universe.

The number of digits in a Googolplexianth is so large that there is no known way to calculate or express it.
Here is a table illustrating the magnitude of increasingly larger numbers (note that a Googolplexianth is not included as it is too large to fit on this chart):
Number Name  Value 

Million  1,000,000 
Billion  1,000,000,000 
Trillion  1,000,000,000,000 
Quadrillion  1,000,000,000,000,000 
Quintillion  1,000,000,000,000,000,000 
Sextillion  1,000,000,000,000,000,000,000 
Septillion  1,000,000,000,000,000,000,000,000 
Octillion  1,000,000,000,000,000,000,000,000,000 
Nonillion  1,000,000,000,000,000,000,000,000,000,000 
Decillion  1,000,000,000,000,000,000,000,000,000,000,000 
Google (10^100)  1 followed by 100 zeros 
In short, trying to imagine how many zeros a Googolplexianth has is akin to imagining the infinite. It is a number so large that it defies comprehension and serves as a reminder of the vastness and complexity of the universe.
Other responses to your question
10100 zeroesWritten out in ordinary decimal notation, it is 1 followed by 10100 zeroes; that is, a 1 followed by a googol of zeroes.
A visual response to the word “How many zeros does a Googolplexianth have?”
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In addition, people are interested
A googol is the large number 10100. In decimal notation, it is written as the digit 1 followed by one hundred zeroes: 10,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.