The weekly recommended amount of fatty fish depends on who recommends it
NHS recommends 1 portion (around 140g) of fatty fish per week (plus 1 portion of non-fat fish), some other sources advise 3-4 portions. If you do not eat fish for some reason, you need supplements. Simples!
Mercury contamination is the main reason you should be careful about overconsumption of fatty fish, especially big fish. The best example is tuna – while it is hugely popular among healthy-eaters and contains much protein and some Omega-3s, it can be highly contaminated with mercury. Therefore eating tuna regularly is not advised, whereas mackerel is low in environmental contaminants and can easily be eaten more than once per week on a regular basis.
Here is a reliable source of knowledge about mercury contamination: https://www.scientificamerican.com/article/how-does-mercury-get-into/
Contamination is also the reason why it is better to buy more expensive, reputable brands of fish oil supplements – they remove heavy metals, pesticides, solvents, PCBs, and other contaminants from their products. Again, if you do not get enough Omega-3s with diet, you’ll benefit from supplements.
My favorite brands are Designs for Health, Thorne Research, and Solgar.