I think it's personal preference, yours as well as the rats,
and also depends on the size of the rats
you can get more stuff in a cage with baby rats, once they are bigger, they may need some stuff removed to make room for their bigger bodies to get around
some rats adore their hammocks and want as many as you can cram in there, others, like the girls I have now, dont' seem to care for htem that much (i'm shocked to be honest) They prefer things on the floor and shelves more than hanging stuff
watch your rats and see what they like and what they use and then adjust the cage according to that.