Not that many. Power from Poland or Germany is not that common in Sweden, Sweden exports most of the time. It can happen during periods of low winds, but it's not that frequent.
It has become far more frequent than it used to after the closure of Ringhals 1 and Barsebäck, two nuclear power plants in the south of Sweden. The power grid in Sweden is designed to take electricity produced mostly by hydroelectric plants in the north and middle and nuclear in the south. The closure of those nuclear plants in the south has led to an imbalance, especially given the increase in power use in the south. To make matters worse the "government" - between quotes here because they hardly deserve that term - allowed the establishment of several data centres by the likes of Amazon, Facebook and Google, each of which consumes enough electricity to make it impossible for other industry to be established in the same area given the limits of the distribution network. To make things even worse those data centres got a really sweet deal on electricity, they're not paying all the taxes and surcharges which other consumers and industry have to. In return they provide a couple of dozen jobs, each of which was subsidised by the Swedish state (in addition to the mentioned deal on electricity) to the tune of 2.5 million SEK (around $275.000,-).
Here's some more choice words on the failures of the current Swedish energy policy:
