SignalTap II Logic Analyzer: Maximum acquisition clock frequency - where to find?

Signatap is built out of logic resources so it's maximum frequency really depends on the design. If you exceed the Fmax of what the signaltap logic can handle then it'll show up in Timequest as a timing violation but there is no predefined Fmax in general for it. Whatever sampling clock you use is what you want to report to see if signaltap has any timing violations but judging by your screenshot you don't have a violation assuming that sample clock is properly defined in timing constraints.