Someone googled for exactly this phrase and found this blog. I will not ask ‘why, oh google, why?’ instead, I will try to look at the issue a bit. The answer of course is: it depends. And in the case you do not do a language like Toki Pona, which has a fixed set of vocabulary as part of its design, the answer will probably be ‘many’! I have more than 1700 words in rejistanian (not all are stems), but still I feel that I am not finished. Sure, I can say things like that there is no fixed order in which the home team is listed in traditional rejistanian sports, or that the train was too late and thus I was unable to come on time, but I am not sure that I could talk about everything I talk about in real life in rejistanian. I know that if I was confronted with a malfunctioning car in Rejistania, I would not be able to understand what exactly was wrong with it only based on the description of the mechanic. I know that I would not be able to ask in a rejistanian cosmetics store which foundation, eye shadow, lip stick, concealer, etc, they would recommend because none of these words exist. So from personal experience: more than 1800 words. Of course, it depends on the language. A language used by stoneage tribes needs no word for carburator or gasket. A language used by aliens with tentacles (like the Rikchik) needs no word for finger. It also depends on what a language is supposed to be used for. A naming language will not require more than about a hundred words. A language which will only be used for a specific purpose only needs the vocabulary for this purpose. However, when you want to rickroll people, ask the referee about the location of his seeing-eye dog, order food in a restaurant, discuss the latest election, tell about that new band you discovered or convince people that Bielefeld does not exist and what is really there, then you need word, lots of them.
So, while I cannot give numbers, I can tell people that the only way to deal with the creation of vocabulary is to grin and bear it. As soon as a basis is done, there will not be the need to create words all the time. If the language has a clear purpose, it will reach the point where you can see a sentence and immediately know that you coined all its words far quicker, mostly because you have a clear direction into which to direct your effort without being distracted by attempting to explain to people that Bielefeld does not exist and what really is there in your constructed language. It also helps to have lots of ‘Unabsteigbarkeit’ in your language, ie the ability to create a word from affixes (intolerability is a good English example), as well as much compounding but not a cure-all. Some compounds make little sense unless you remember the reason behind them, so they have to be documented just as well.
I guess my significant other has a much more laconic way to answer the question though:
(Rejistania) In the category ‘who googles such slani and finds the RWotD’, the prize goes to that person who googled ‘how many words does a conlang need?’
(Allanea) the answer is simple
He is perfectly right. I can imagine that even after a century of using and improving rejistanian, the future me will find new lexical gaps. And IMHO that is a great thing.
eljanicator provided some numerical values, which probably work well as ballpark numbers:
I don’t remember where I read it exactly, but I’ve heard that a very limited special-purpose language needs at least 100 words, a trade/diplomacy pidgin needs at least 500, a fully functional language for everyday communication in a wide variety of subjects needs at least 2000, and most modern-day real-world languages have at least 6000. Many have considerably more. The very “largest” ones have a few hundred thousand, though in that case most people who speak it only actually know a small fraction of the total, as the bulk of the language consists of highly specialized or exotic words that most people don’t really need or encounter in ordinary life.
The word of the day for today is ytanu’het which means neck or rather the same as the German word ‘Hals’, ie: everything between the level of head and shoulders.
Example: Ytanu’het’xe mi’tore. (neck-GEN1S 3S-hurt: I have a sore throat)