An estimate based on the Ethnologue data.
The Ethnologue (sourced from Wikipedia) lists languages with more than 50 million speakers. Most of these can be easily classified as either using A or using 文. There are some edge cases. Russian uses the Cyrillic А, which is visually indistinguishable from the Latin A. Korean uses Hangul for most writing, but Hanja (Chinese-derived characters) are used in some contexts. Javanese has a local script, but is widely written in Latin script, Hausa also is written with the Latin (Boko) script. I've included Husua, Javanese, Russian and Korean as languages that use A or 文.
That data covers 4.276 billion people, or slightly more than half the world's population. Of those people 72% speak a language that uses A or 文.
Among the rest of the world, one might expect a similar ratio. Among the languages with fewer native speakers there are many that have adopted Latin script, at least partially. There are also several Chinese languages that are not accounted for here. On the other hand, there are many languages of South and South-East Asia that don't use either.
This account also doesn't consider rates of illiteracy (both child and adult) And one should really take more care with languages that use multiple alternate scripts.
However, it seems reasonable to say that 60-70% of the world's literate population would recognise one of the characters as a symbol used by speakers of their language.
The next most popular is tricky because many scripts are related but have small differences. An Arabic character has very wide recognition, not only in the various forms of Arabic, but as an script used in Africa and many parts of Asia, perhaps optionally.