Hm I know NaNoWriMo used to be a thing but for one reason or another it fell apart (IIRC it was a huge controversy but sadly im not deep enough in the writing community to really know or recall)
Do you mean challenges or competitions? If you're just looking to challenge yourself to write X amount in a certain number of days you can do that on your own at any time. In fact, you could probably host one yourself
But in terms of actual competition with like prizes or awards, I'm not sure? I think Tapas does the occasional one but I'm not a writer so I'm not super plugged into those events
