Ask HN: Can a single LLC offer multiple unrelated services?

You can do anything you want business-wise under a single LLC, though the IRS wants you to identify the primary business category you're in, they really just want statistical info, it does not prevent you from doing other types of work under it. The point of separating into multiple LLCs (or S or C corps) is entirely to isolate the risk of financial ruin including bankruptcy. If a separate LLC fails, goes bankrupt, gets sued, it won't impact any of your other LLCs unless you've intermingled accounts and pierced the corporate veil, so to speak. If you're doing anything that requires a professional license such as health care, engineering (building bridges), and so on, you'll want to get advice from the experts, but NOLO guides would be a good start.