Multiclass BDT, number of Test/Training events

TMVA
2

It depends on how you name your input trees.

If you add trees like this:

AddTree(treeClass1, "inputClass1", ...)
...
AddTree(treeClassN, "inputClassN", ...)

You need to call:
PrepareTrainingAndTestTree("", "nTest_inputClass1=5000:(...):nTest_inputClassN=5000")
I.e. specify the events for every class.