There’s a hard limit of 20,000 list items retrieved in a search (which we can raise on dedicated installations.) There’s also soft limits after 7 - 10 minutes for any given workflow run, which you’ll probably hit before the 20,000 list limit (we can’t raise this one on dedicated right now; we’re working on various things to get rid of this limit, including speeding things up, and breaking up long-running workflows into chunks so we can run them persistently).
Running a “Schedule API workflow on a list” action on a search starts scheduling items one-by-one; if you hit a limit, it’ll be in the scheduling phase. (Once a workflow is scheduled, it’s pretty much guaranteed to run).
My general recommendation is if you need reliable operations on large lists of things, use search conditions to break them into batches of 100 or so. Over the next 6 months we’d like to get to the point where this just happens automatically behind the scenes and you don’t need to think about it, but there’s a lot of technical challenges around doing that.