Can someone write more than 5,000 words per day? If so, how much practice is needed to develop this skill?

Yes. You only need to have something to communicate in words. I write in a journal most everyday and usually only write 1000–1500 words. But when writing novels I have gotten over 5000 words a day. Not often, but sometimes I’m on a roll, so to speak. I have all day to write since I am retired and in assisted living. Start at five AM and finish around eleven PM. with food and drink breaks, and other breaks of course. I write at least 1000 words almost every day. This is what I do. It keeps me focused and consciously alive. I do have friends. When I see them or speak with them, sometimes I write less. Friends are important to me. Reading is important to me too. And, I like Asian romances and documentaries on Netflix. The news too, of course, but once a day is enough. Usually I read the newspaper in my room from the front desk. My next door neighbor and I share it. He’s 99 and I’m a young 82. Growing old can be fun and interesting if you can let it be (even with growing older pains). Have a great day. Time for bed.