With 12 languages -Mandarin, Spanish, English, Hindi, Arabic, Portuguese, Bengali(Bangla), Russian, Japanese, Punjabi, Javanese, German- you can communicate with at least 50% of the world population, talking in their native language.
The problem with higher percentages, is that the share of speakers decreases exponentially. For each additional percentage point you would be adding more and more languages. To approach the 75% figure, you'll need to add languages like Bhojpuri, which cover a mere 0.43% of the world population.