Use a stack rule. Basic sets how many players and which teams. Advanced sets exactly which positions.
If what you care about is which stack types you play, such as QB plus two, the Stack Exposure column is the easier tool. A stack rule is for when you're opinionated about which positions can be inside those stacks, like QB plus two with no running backs.
Open Rules, click Add New Rule, and choose Stack.
Basic sets at least or exactly how many players from the primary team, which positions count, which teams it applies to, and whether to force a bring-back from the opposing team. In NFL or CFB, note that as long as a QB is checked in the rule, the rule will always feature a QB in the stack.
Advanced gives you an operator for every position: equal to, at least, less than, or a range. A QB who's never stacked with a running back is RB exactly 0, WR between 1 and 2, TE at least 0. Defense defaults to 0. You can set the opposing team's positions the same way.
Both work the same way with the sims. SaberSim already builds stacks from what happens in the game simulations, so a stack rule shapes the ones it builds rather than creating them from scratch.
Related: How do I control which players can be used as one-offs in my lineups? · How do I exclude the stacks I don't want? · How do I avoid playing multiple players from the same team without the QB?