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I wouldn’t push it beyond 2 until you reach 25 (assuming you are male, if you are female you shouldn’t exceed 2 until you don’t want to have more children) and that assumes you are talking about the ~150g cans. Your prefrontal cortex continues to develop until 25 and the most damaging aspect of mercury is that it disrupts brain development.
There are better choices without the mercury risk. Sardines taste better then tuna and contain large amounts of EPA & DHA (omega 3’s) which have profound benefits for your cardiovascular and brain health. Canned salmon is also good, because its safe in large amounts you can also get it in much larger cans to save money.
Sardines & salmon are safe to eat a couple of portions a day of. Even if you are among the ~10% of the population who are susceptible to uric acid issues the purine isn’t a problem if you hydrate well. Shellfish are also safe if you want to throw some prawns in to the mix. Oysters are insanely rich in zinc, 3 of them a day and you hit your RDA.
I recommend sardines, mackerell and so on. The smaller the fish the better. Here’s a cool guide. https://www.nrdc.org/sites/default/files/walletcard.pdf
I also don’t recommend eating salmon if its grown/farmed. Antiobiotics and all kinds of horrible conditions.