Let me see if I can lessen your confusion a bit. Campden tabs are metabisulphite (either sodium or potassium - if you can tell what you're buying, potassium is better). Metabisulphite adds something called SO2 (sulphur dioxide) to your must. Sulphur dioxide, in sufficient concentration, will kill yeast cells, along with lots of other spoilage organisms. The molecular form of SO2 dissolved in the must liquid is often simply referred to as "sulphite," although that can be a misleading term. But for simplicity I'll use it interchangeably with SO2 from hereon out.
What makes things complicated, are 1) the amount of "free SO2" added to your must will depend on its pH, and also on what other chemicals are present in the must that can bind with SO2 and render it useless for sanitization, and 2) different strains of yeast are susceptible to different concentrations of SO2. In general, wild yeast strains (and many spoilage organisms) can be completely killed off by as little as 50 ppm sulphite. Campden tabs are sized such that they add about 50 ppm per tab, per gallon into a must of pH 3.5 (sort of an "average" pH for a wine must). So if you have one gallon of mead and you want to add 50 ppm sulphite to it, you pop in one crushed Campden tab. HOWEVER, some of that sulphite will be bound with other chemicals (in general), and some of it may not be free, depending on your must's pH. So, in general, it doesn't hurt to add a bit more just to be sure.
Further, commercial wine yeast strains have been bred to be more sulphite resistant than their wild cousins. Some strains are very resistant; they can tolerate as much as 325 ppm sulphite, and at those levels you can even smell and taste the influence of the chemical in your mead. So, (also in general), don't rely on sulphite additions to stop an ongoing ferment that you've started with a commercial yeast strain. It likely won't work, unless you are willing to have your mead smell like a burnt match.
Again in general, additions anywhere from 50 to 150 ppm are undetectable in the mead, unless you are particularly sensitive to the sulphite (i.e. allergic). Most people are totally unaware that sulphite is present in a mead or a wine, even up to that 150 ppm level or beyond. Whether you ever want to have that much sulphite present in your mead, is totally up to you.
