It is sometimes said that English has more words than any other language. Nobody knows exactly how many words there are in English -- it's impossible to count them -- but it's around a quarter of a million, although some estimates put it at 1 million. Spanish probably has about 100,000 words, although I haven't been able to find a reliable source.
One reason English has so many words is that over the centuries it has borrowed many words from all sorts of different languages, but often kept the old ones. Thus there are often two or even three words which mean almost exactly the same thing: "sick" and "ill", for example, or "bold" and "courageous".
Why do estimates vary? Well, it depends partly on whether you count words like "knackatory", which means "a place for buying knick-knacks" but is not used by anybody today. How old-fashioned does a word have to be before you discount it? And what about dialect (e.g. "bairn" means "child" in some eastern parts of England and Scotland)?
Whether English really does have the largest vocabulary in the world is another controversy. Languages like German, for example, allow you to form new words simply by sticking two or more words together:
Autobahn = motorway/freeway
Anschluss = connection
Stelle = place
Autobahnanschlussstelle = motorway junction / freeway intersection
Such languages have, theoretically at least, an infinite number of different words.