You can also do a For Each loop (first pick the relevant instances by preferred method) - Set maxvaluevariable to max(maxvaluevariable,object.value) or Set MinValueVariable to min(minvaluevariable,object.value).
This will basically go through each picked instance and if the instance value is higher (or lower) than the previous highest or lowest, it will update the variable. Otherwise, the last value stands.