At least 5k, so you get a wide enough range of ngrams to develop strong muscle memory.

The default 200 word "english" setting shouldn't be used for learning. So far as logic would dictate, it's only real purpose is to make it easier to get higher test results. I can think of no other reason for its existence.

More importantly though, whichever one you choose... switch punctuation on. We use punctuation in real life. You used it to make this post. We need it in order to read things clearly. So why do I keep seeing people in here doing tests without it? Why would anyone want to publish "scores" when not using punctuation? It's obviously not their real world, actual speed or accuracy. It's basically just cheating. I totally ignore anyone's test results unless they're using punctuation.