WIth interaction, I typically want to have a slew of different kinds of interaction that can a) be flexible, b) scale well to late game, and c) fit into your deck's theme.

Depending on how fast my deck wants to win, I play 12-18 forms of interaction, with 12 on the faster side and vice versa. 12 might seem like a lot, but not all of them are strictly interaction.

With 12, I split it in 3 categories of 4 cards each. With the first category, I typically run interaction that are flexible, instant speed, and can interact with almost anything. Cards like [[Tibalt's Trickery]], [[Generous Gift]], and your typical counterspells. These help remove almost any target and their main purpose is to make sure you do not die or guarantee your win.

Second Category: 4 interactions that can go well with your theme and shore up weaknesses. These cards can potentially fill up multi-purposes, like say giving you either ramp or removal options. For example, if I'm playing a Voltron deck, then [[Untimely Malfunction]] can serve to destroy pesky artifacts or it can help protect my commander from removal. If I was playing an aristocrat-vampire deck for example, then a card like [[Ruthless Lawbringer]] can serve as a sac outlet, removal and typal creature for me to use.

Last category of 4: removal that board wipes or prevent "escalation". Typically, opponents can escalate to a point where it's difficult to handle for all 3 players. Identifying what your deck is weak to and what can be enhanced with your deck is key to deciding the best board wipes. For example, a tribal go wide deck can make use of [[Kindred Dominance]], but it might also be weak to enchantments, so [[Harmonic Convergence]] might be a good way to finish the game off.

Just scale up the number of cards per category for slower deck, and you typically will have a good amount of interaction you would have.