"How many scripts would I need to learn to read every language?"
I think this is an interesting question and I would like to take the time to answer it in a few different ways. Now, this is vague, but I will answer it the best I can. For those who don't know, a script is a system of writing down a language.
So, firstly, just because you can read a script doesn't mean you can read the language. You might know how to pronounce ελληνικά just by knowing the Greek alphabet, but unless you actually study Greek you may not know that this is the word for "Greek". But for the sake of argument, let's assume you have the magical ability of understanding every single language spoken today, just to make it easier to answer. In this scenario, you can understand every language you hear, but you are illiterate and want to fix that. What should you do?
Now, assuming you want to read every language in the world, let's talk about what you mean by "every language". Every major language? Every language with a certain amount of speakers? All living languages with a script?
1.) I want to read the top 50 most spoken languages in the world
I'm assessing this question according to this link's list of most spoken languages since 2017. Minimally you would only need to know about 20 scripts to communicate in the languages on the list through writing (Latin, Devanagari, several unique scripts in India, a script from Indonesia, Chinese logographics, Hangul, all three Japanese scripts, Cyrillic, Arabic). Keep in mind that some languages can use more than one script and not all languages use exactly the same letters even if they use the same script. And the list can change depending on statistics, year and resources.
2.) I want to read scripts from languages with over 10 million speakers.
You wouldn't have much more difference, with somewhere between 1 and 5 scripts added to the list depending on the resources (usually another alphabet, one of the easier kinds of scripts to learn). Many languages had scripts adopted because of expansion, colonization or cultural interaction in general.
3.) I want to read all of the spoken languages in the world!
Here's what the site Omniglot has to say....
4 Abjads are currently used.
27 alphabets are still used.
70 "syllabic alphabets" (abugidas) are still used every day
8 syllabaries are still used
2 unique logographic systems are still used (Chinese and Japanese), but another is occasionally used for religious and decorative purposes (a Chinese indigenous language known as Naxi)
In total, this is around 102 scripts you would need to learn to read every single language still spoken in the world today in every possible way it's written... That's a lot!
Also if you consider sign language a spoken language (spoken being with the hands) there are some scripts developed for that: https://aslfont.github.io/Symbol-Font-For-ASL/ways-to-write.html
Question for you: Do you have a favorite script? Would you learn the langauge(s) that use it? Why or why not?
I hope you find this post informative!
Don't forget the scripts for writing in sign languages: https://aslfont.github.io/Symbol-Font-For-ASL/ways-to-write.html :)