Hi Sengyan! You can put as much fish as you like in. It will just make it stronger, the more you put in.
I often don't even take the step of making emulsion. My Dad is an avid fisherman, so I have access to lots of fish remains. I just bring them home, and bury them under my rose bushes as is. They love it!
Here is a more complicated recipe for fish emulsion if you would like to try it. I have never used it, so cannot vouch for it.
You can use fresh fish parts or any canned fish. The juices, sauces, or oils in the can can be used to breed beneficial microbes and supply extra proteins in the tea, so use it.
If you do use canned fish products, you may want to let it decompose mixed with some finished compost, good garden soil, etc. in a container for a few days before using. Most canned meat products contain preservatives, and this ensures that the good microbes in the tea will not be killed off or harmed in making the emulsion.
If you use fresh fish, you need to compost it in a 5 gallon closeable bucket. Fill bucket 1/2 full with extra browns like sawdust, leaves, or straw. You can add some molasses to the fishy mixture in order to build up microbes to speed up decomposition. Stir the bucket daily or every other day in order to get air in the mix for better decomposition and better aerobic microbial growth in the emulsion. Let this paste rot for at least 1-2 weeks.
If you like you could add 1-2 tblsp of Epsom salt to the mix for extra magnesium and sulfur and for extra trace elements you could add 1-2 tblsp of apple cider vinegar.
You can apply this fish emulsion at a dilution rate from 1:1 to 1:5 ratio (5 gallons of tea to 25 gallons of water).
Hope this helps you out! Let me know how it goes!