What should be the right texture streaming pool amount of the biggest?

Is there any professional answer that What should be the right texture streaming pool amount f the biggest?
For example: I have a 12gb grafic card and 64gb pc RAM, while i using ue5 but not ue4,refuring to the ue4 official video about the texture streaming pool, judging my 12gb grafic card,the biggest number should be 12gb?Am I Right ?
Then my question is ,if I creat a game use 12gb texture streaming pool,then someone playing my game only have a 8gb grafic card,what should happen when he is playing? Am i Wasting time creating this game while someone can not playing it ?

If a game really needs 12GB of video memory and it’s either high quality or a failure, we need to do some optimizations to put it in “Settings” so that the user can choose their own flow mode