Not very many! Keep in mind that you only have a very short WiFi connection with Tello, so if a stiff breeze starts to push Tello away from you, it won’t be long before Tello gets blown out of range. So I’d say your risk of losing Tello is pretty high in anything other than a gentle breeze.
I have tried the Tello in moderate breeze of 15mph but made sure I was in open space and had fast mode on. Tello did very well but did get blown about a lot and I almost lost it when a gust took it away out of wifi range. Learnt my lesson and got a wifi repeater and now I only fly under 12mph breeze. I find it a lot of fun actually flying it in conditions that people say it should not be flown
Same as robertv wrote
I never ever fly it in winds above 5 mph, and your guaranteed fun.
People had lost there Tello as flying in wind as you really do not know in 20 meters height what is the wind structure up there to drift you fast and away.