And how much you know ?

How many*
There are at least 900 roots, which you can use to create other words, for example:
reto - web, pagxo - page: retpagxo - website.

I've seen the number 5000 thrown around for total official roots, don't know the source though. A book I have says 2500 but it's extremely old, so the 5000 figure is probably closer to accurate. Obviously from that you can construct a lot of words, ESPDIC has around 63k entries atm if I remember correctly.

Considering under 600 words are required to obtain 80% understanding though I couldn't even imagine memorizing that many. No idea how many words I actually know off-hand but it's a few thousand.

I understand that the magazine for young people Juna Amiko uses a vocabulary of about 1500 words. That should give you some idea.

The good news is that you don't have to learn them all at once.

The number of "official" roots is around 2500, I think, once you include the Universala Vortaro and subsequent Aldonoj. This doesn't include many "international" roots.

You can speak very well with just the 1000 most common roots, if you also are familiar with the 40-odd affixes and word-compounding.

Why does it matter how many words Esperanto has? Take a strange language like Englilsh, for example, that has well over 500,000 words - most of which are useless for the average speaker and well unknown to the average speaker. Many of the words in English are technical terms, dialect, slang or obsolete. Then there are the overlaps as like breakfast, lunch, dinner, supper, etc which are all types of meals but which have to be learned separately, the first concerning a morning meal, the second a meal around noon and the third an evening meal whereas the fourth may be substituted for the third according to the dialect spoken. Do you mean morphemes? Or do you mean compound words (like textbook)? If textbook counts as one word, does then lernolibro in Esperanto count as one word? Or do you mean just text and book with the possibility of creating textbook though not text citation? Counting words is a little like counting clouds.
And then we have the problem of politics in English. So for some people "French fries" do not count though "liberty fries" do, like "sauerkraut" vx. "liberty cabbage". Is the expression "pig" one word (an animal with four feet that is made into bacon) or two (also a cop or policeman, especially one who tramples your rights as a citizen)? And is the verb "to pig" counted as a separate word since its meaning is only metamprhically related to the animal? Or when it is used as an adjective as in "pig pile" (what kind of a pile, well, a pig pile - is that made of pigs? No! It usually consists of children though it may consist of American football players).
And in Esperanto are you going to use the PIV or the Reta Vortaro and are the neologismoj going to count or not and also metaphors and idiomatic expressions (like "krokodili" or "gufejo")?
How many words does Croatian have?
Furthermore, I would kindly suggest that you seek help from one of the participants in regards to using English. I will gladly help you iron out some of your texts if you want. It is a tricky language and one often needs some guidance when using it to be well understood (yes, I know, to be understood well, for the speakers of the British variant).

You will find statistics here:
http://akademio-de-esperanto.org/aktoj/aktoj2/bro....

2500 words of common use; the other words are specialized (science, computing, local specialities, etc), the list is open.

Esperanto roots can not be compared to words in other languages. A root becomes a word after adding to it some ending. In this way one root can become a noun, an adjective, a verb, and so on. Number of roots in modern esperanto is about 5000. How many words can be made of this stock? Unknown, a million or more perhaps. Remember that new words can be created by combining different roots together as well: stelscienco (astronomy), discienco (theology)... So, what is a new word in other languages, in esperanto it is just a shade of the root. The esperanto dictionary is multiplicative - adding to it one new root multiplies it by all shades of affixes and sensible root combinations. I would answer the question with: "Esperanto certainly has more than enough words for all imaginable uses".