Do a search for all the new words that entered the language in the last century. Select first quotation from the drop-down box. Then, in the box marked Date of entry, type 1900-1999. This gives a list of the words for which the first recorded use is in the twentieth century.
How many new words?
Let’s examine the rate of growth of the English lexicon over an arbitrary period of time. This will involve performing a series of searches similar to that above, because in order to get a thorough picture it is better to limit the date ranges to, say, a decade at a time.