Rad counters are a kind of counter that a player may have. They’re not associated with any specific permanents.

If more than one player is instructed to mill at the same time (by the activated ability of Raul, Trouble Shooter, for example), as long as at least one nonland card is milled, Screeching Scorchbeast’s last ability will trigger just once.

Rad counters don’t go away as steps, phases, or turns end. They only go away when an effect instructs a player to remove rad counters from themselves.

If a player has fewer cards remaining in their library than the number of rad counters they have when the triggered ability resolves, they’ll mill as many cards as they can.

“Do this only once each turn” lets you choose whether or not to create the appropriate number of 2/2 black Zombie Mutant creature tokens as the triggered ability resolves. If you choose not to create tokens, the ability will trigger again the next time the condition is met. Once you choose to do so, the ability will no longer trigger for the rest of the turn.

There is an inherent triggered ability associated with having rad counters. This triggered ability has no source and is controlled by the active player. The full text of this ability is “At the beginning of the precombat main phase of a player with rad counters, that player mills cards equal to the number of rad counters they have. For each nonland card milled this way, that player loses 1 life and removes one rad counter from themselves.”

Keep track of how many rad counters each player has. Potential ways to track this include writing them down on paper or using dice, but any method that is clear and mutually agreeable is fine.

In a game using the shared team turns option, such as an Archenemy or Two-Headed Giant game, the inherent triggered ability associated with rad counters triggers once for each player on the active team that has rad counters. Each instance of that ability is controlled by one of those players.

The cards are milled all at once, which means abilities that trigger “whenever one or more nonland cards are milled” will trigger exactly once as long as at least one nonland card was milled.

Any effects (such as proliferate) that interact with counters a player gets, has, or loses can interact with rad counters.