I just started fishing myself a little so correct me if im wrong

From my experience it depends on your fishing proficiency and the type of fish how many you can stack in total.
The higher your prof, the more you can stack.
The lower the fish level, the more you can stack.
( There is a chart you can pop up here :
http://ladderslasher.d2jsp.org/index.php?g=7# )
Currently im at lvl 1 fishing and was able to stack 30 sardine fish.
They were all cooked before and the life/cook progress of the stacked fish
should be the average result of the life rates of all fish you stacked together.
Therefore it might be helpful to add only fish with a good life amount not to ruin the total result too much.
I e.g. accidentally added a 50 mana fish to the 20 life fishes and soon the total result went down a lot

This one might also help you:
http://forums.d2jsp.org/topic.php?t=31971461&f=272