The FDA recommends a maximum daily sodium intake of 2300 mg, so that’s about 70g per month.
Assuming salt is fairly close to pure sodium chloride or NaCl (ignoring the microplastics!), then about 39% of the weight of salt is sodium (23/58.5 based on average atomic weights of Na and Cl). Therefore a 240g box of Malden flakes is about 94 g of sodium.
So according to my math, a box per month is more than you want to eat, but not by a crazy amount, especially if some of it goes down the sink with the pasta water.
But of course that assumes that all your salt intake comes from this box and that you never eat anything else containing salt, which is completely implausible for most people.
Also, note that 2300 mg/day is a recommended maximum, not an ideal target, and critics will argue that government guidelines are too high because of lobbying pressure from the processed food industry. The American Heart association recommends an ideal sodium intake of 1500mg/day. That’s about half a box per month.