I have a 50mbits line and the pc arrives at 48 mbits.
how many kbits should I set at obs that the stream runs smoothly?
The stream is sent via the upload line, which has nothing to do with your download rate.
First, do a speed test to determine your upload speed.
You should not completely exhaust your available data rate with the stream, as there can always be a breakdown and other programs also want something from the upload.
Depending on how fast your line is, you should adapt your bit rate to the resolution and frame rate used.
Twitch recommends a bit rate of 4,500 kbit / s at a resolution of 1920 x 1080 pixels and 60 FPS. Anything below that would make the picture look mushy and out of focus.
It doesn't hurt to set more than necessary; However, the viewer can experience stuttering if their Internet connection is insufficient or the device can't process the image as quickly.
https://stream.twitch.tv/encoding/
Have an 11 upload what would you recommend?