Not currently in Poland. There were attempts to do so though. All in all certain presumptions can trigger red flags for the tax office, all and any of the following: 40h a week, paid days off (especially 20/26 days), work in the same place, at the same time, under supervision. With IT its safest to contract for a company without entity in Poland. Also you’ll find many people who still cover a lot of the above and never been bothered by the tax man. So I’d say its just being extra careful.

Not in all of EU: at least in Finland, the tax office clarified a long time ago that if the papers (business registration, contracts, insurances etc.) are done properly, the number of clients doesn't matter.