6000 words for college entrance examination, 8000 words for TOEFL, 3000 core for IELTS. Similarly, the network containing massive data also has corresponding high-frequency words.
The 1/3 million most frequent words
google collects network data, and publishes Google Web Trillion Word Corpus. [Peter Norvig] (https://norvig.com/ngrams/) summarizes 1/3 million most frequent English words from it. I then select 30,000 words by frequency , Then use the proper API to translate them all. See the link at the end of the file.