I'd go with 7000–8000 words. Learning 3,000 words is not tough. I've been studying Latin for a year and have about 3000 words. The objective is to keep the average proportion of unfamiliar terms under 1%. If you want to become fully fluent, you'll need to learn about 10,000 words. When you learn Latin, it's useful to think of it as learning new languages. Each word has an English equivalent and sometimes more than one English equivalent. To know which option is best, choose the one that seems most appropriate for the context. For example, instead of using ferro, you could also use кирпич or запорованный. You can pick and choose among these alternatives depending on what kind of material you are reading or listening to. As you learn new words, it's important to understand their different functions in sentences. For example, words like quid, cui, autem, and unde are all interrogatives. They all ask a question and give you a possible answer. Knowing this about them will help you decide how to use them in your own writing. Another important thing to remember is that Latin is an inflected language. This means that each word belongs to one out of several possible forms based on its part of speech.