If you distinguish publishing (clicking the button on a finished hub that says "publish now") from writing, then there is no theoretical limit. You could write a hub a day for a year, then hit "Publish Now" 365 times.
If you mean producing a hub from scratch and publishing it, it depends on what kind of hubI you want to publish, and what you're goals are.
If our goal is to increase income, here are some thoughts. (Note, I'm changing the question: "How many can?" is about maximizing speed. Now, I'm talking about optimizing revenue.) A few years ago, HubPages encouraged challenges like 30 hubs in 30 days - one hub a day. Now that the Internet is calling for higher quality content, they no longer recommend this. They encourage a maximum of 15 hubs per month, and added the guidelines (goals) in the upper right of the HubTool.
I've just stabilized my own production schedule, and I'm starting to calculate production timings. I can produce a short hub (2,000 words) that meets stellar standards except no video in about 5 hours. Check out my hub on Pangrams to see the result. On the high end, I didn't count, but I would guess that it took me about 20 hours to produce my Stellar hub on the Battle of Britain (7,500 words), as that included a lot of research, some in-depth analysis, a fair amount of hunting for images, and a lot of fact-checking.
Down the road about 6 months, I'm hoping to compare income-per-hour of production work across simple hubs, stellar hubs, and video hubs. (I haven't started on video yet.) But I've got to get a good sampling of hubs matching each standard up, and then let them mature.
If you're not familiar with the definition of a Stellar Hub, read here: https://hubpageshelp.com/content/Learning-Center-E...