Ask HN: Outstanding packets calculation in Go-Back-N ARQ when ACK lost?

https://imgur.com/a/ENjOgBG

Assume window size=3.

Assume ACK#3(i.e. for the packet#2) is lost.

Now how many outstanding packets can transmitter send?

I mean, can it send pkt#3,pkt#4,pkt#5? What determines it?