TL;DR There's no exact, right, or wrong amount. It depends on your decks needs for frequency of seeing interaction as well as meta considerations.

I prefer not to think about counters in terms of how many in raw numbers but rather how often/when do I want them. You can apply the same logic for any card function, e.g. tutor, board wipe, x outlet.

In [[Baral]], for example, you probably always want 2-3 in hand, so 30 might make sense. In a deck with kill spells or spells that answer resolved threats, the number will probably be much lower. If you are a combo deck and want protection in your starting hand, think along the lines of 8 cards/99 in the deck and the likelihood that one is interaction. You can add/subtract what you want to fit your desired frequency of seeing a card.

In terms of meta, it might also depend on what kind of decks your opponents pilot. If you have a dedicated combo player at the table, they might run a fair amount of protection and you may have to adjust accordingly. If they run very oppressive key pieces, it might be another reason to run counters/removal. If it's just battlecruiser magic or value/good-stuff piles, counters may not accomplish as much as you might like and you might opt for boardwipes, instead.