PNG are image files and as such do not have a concept "pages". The image can contain multiple individual SAS graphs depending on how they are made but you would have to share a lot more detail about what you have and what you are attempting.
I'm not 100% sure what you're asking, but here's one way to use 1 proc sgplot, and produce multiple graphs (with each graph being in a separate png file - one for each value of the 'by' variable)...
proc sort data=sashelp.class out=my_data;
proc sgplot data=my_data;
scatter y=height x=weight;
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