How many apps are in the Apple App Store or Android Playstore?

My advice on answering this estimation question in a Google PM interview:

Clarifying Questions

App store contains generally 4 catrgories - Games, Apps, Books, Movies, Audios (Songs & Podcasts). Are we focusing on only apps or are we considering games are apps too.

Solution

Considering app stores host many apps, these apps are divided into various categories. Top categories are listed below.

Considering there are almost ~200  countries in the world and almost 40% population is using smarphone now. We need to create an equation for each categroy listed below

Total Coutries = 200 

Countries with Internet / Smartphone infrastructure as well as users = 150

Hence for a given number of categories below. we will have country specific variants of simiar apps. 

Example - Uber in USA while OLA in India and DIDI in China for ride hailing.

Additionally, these apps overlap geography as well. hence we will list down unique apps ideas below and take n% of them as universal apps while 100-n% being replicated in all countries with localization, culture etc.

1. Camera / Photography apps 

• Total unique ideas = 2000 
• Universal app = 20% = 400
• Country Specific Variants = 1600 * 150 = 240,000
• Total = 240,400

2. Social Media and Communication Apps

• Total unique ideas = 1000 
• Universal app = 20% = 200
• Country Specific Variants = 800 * 150 = 120,000
• Total = 120,200

3. News Apps

• Total unique ideas = 500 
• Universal app = 20% = 100
• Country Specific Variants = 400 * 150 = 60,000
• Total = 60,100

4. Exercise Apps

• Total unique ideas = 400 
• Universal app = 50% = 200
• Country Specific Variants = 200 * 150 = 30,000
• Total = 30,200

5. Productivity Apps

• Total unique ideas = 500 
• Universal app = 80% = 400
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,400

6. Utility Apps - Electricity / Water / IOT / Food delivery etc

• Total unique ideas = 100
• Universal app = 0% = 0
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,000

7. Finance and Banking related apps

• Total unique ideas = 500 
• Universal app = 20% = 100
• Country Specific Variants = 400 * 150 = 60,000
• Total = 60,100

8. OTT Streaming apps

• Total unique ideas = 400 
• Universal app = 50% = 200
• Country Specific Variants = 200 * 150 = 30,000
• Total = 30,200

9. Video Calls Apps

• Total unique ideas = 100
• Universal app = 0% = 0
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,000

10. Transport related apps

• Total unique ideas = 100
• Universal app = 0% = 0
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,000

11. Travel Apps

• Total unique ideas = 500
• Universal app = 10% = 50
• Country Specific Variants = 450 * 150 = 67,500
• Total = 67550

12. Fashion Apps

• Total unique ideas = 400 
• Universal app = 50% = 200
• Country Specific Variants = 200 * 150 = 30,000
• Total = 30,200

13. E-Commerce Apps

• Total unique ideas = 100
• Universal app = 0% = 0
• Country Specific Variants = 100 * 150 = 15,000
• Total = 15,000

14. Miscellaneous

• Total unique ideas = 10000
• Universal app = 0% = 0
• Country Specific Variants = 1000 * 150 = 1500,000
• Total = 1500,000