How many grams is "1 can" of beans?

In my experience, a 400 gram can of beans contains about 240 grams of drained beans. That will vary slightly by the type of bean.

It's not what I would call a well-written recipe. Also, a slight variation in the amount of beans will probably not be a big deal for the final prouct. However, given the information you provide, I would suggest that in this case a "can" refers to your 398ml can. Use them all.

It looks like Rose from Cheap Lazy Vegan is Canadian and living (or at least lived in 2015) in the UK. Since the recipe does not state what type of beans, I would assume that if she is in North America she would be using either black beans or kidney beans. The Goya brand usually sells their beans at a net weight of 439 grams. If she is in the UK and using baked beans, most likely she would be using Heinz Beanz which would be 415 grams net weight. Both of these weights would include the sauce.

Since your can would either be a bit smaller or a bit larger than these standard sizes, I would recommend following the recipe and then adding the oat flour as you go. You may need to add more or less oat flour to get to the patty consistency.

One can of beans is 9 weight ounces or 255 grams.