Not that many. Due to the reduced usability of the Jolla compared to the N900, I use it basically only as a web browser (as it has a bigger screen and longer battery life than the N900).

However, on the N900, with only a quarter or Jolla's RAM, I habitually have 8 to 12 windows open at the same time. A few Leafpad instances, a terminal or two, an email, three browser windows... The phone can become sluggish if I keep them open for a few days but it never forcibly closes any application behind my back.