First clarify if the question is referring to individual computers or servers or both. Also does a tables count as a computer?

Breaking down the personal computer first
Approach: Number of employees x person computer per employee

Ask if they will tell me number of employees. If not them ask for Google’s revenue last year – I will assume 100 billion if no info is given. Then get to a revenue per employee number — to estimate that assume 200k average cost per employee and Google gets a 10x return per employee so 2 million revenue per employee.

so 2 billion / 2 million = 50k employees
total personal computers = 50k x 1.1 (assume spares and test devices account for .1 additional per employee) = 55k computers

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if addressing servers

I’d break this down into google used servers for core applications and services and google cloud servers used by other companies

approach 1:( total data on available on the internet + total data unique to google (maps & youtube ) x terabytes per server = total servers needed x redundancy factor = total servers globally

Step #1 – internet achieve servers
Estimating total internet size:
Lets break websites into 3 sizes
Small: assumed size 100 mb (blogs, simple small business websites, etc)
Medium: assumed size 10 gb (corporate sites, medium e-commerce sites, etc)
Large: assumed 5 tb (nytimes, yelp, etc)

Lets assume there there are 1 billion sites and the distribution looks like (70% small, 25% medium, 5% large)
Small = 700 million small sites or 70,000,000,000 MB (70 PB)
Medium = 250 million medium sites or 2,500,000,000 GB (2.5 EB)
Large = 5 million large sites or 25,000,000 TB (25 EB)
total ~27.6 EB

Assume 5 TB per server

Step #1 = 27,600 TB / 5 TB = 5320 (round to 5000 for simple math) to servers to store internet’s data

Step #2 — google unique data servers
Going to just estimate a number for the sake of being lazy but would follow a patter like miles mapped X photo per mile. Youtube minutes of video uploaded per person (break into professional and amateur) X (MB per minute for each bit rate of video stored)
Assumed 5 EB for google maps and 10 EB for youtube
Step 2 = 15,000 TB / 5 TB 3000 servers

Step #3 – total servers

5000 internet achieve + 3000 google unique data 8000 servers

Step #4 – redundancy

Lets assume each of these servers are backed locally 5x so 40,000 servers in each datacenter

Assumed they have this data in data enters in 15 regions around the globe

40,000 x 15 = 600,000 servers