Is there any build-in function in VBScript to repeat a string N-times?

No, nothing built in. Instead:

n      = 5
str    = "Ab"
result = replace(space(n), " ", str)

For a general simple solution

Function RepeatString( number, text )
    Redim buffer(number)
    RepeatString = Join( buffer, text )
End Function

But if the text is short but the number of repetitions is high, this is a much faster solution

Function RepeatString( ByVal number, ByVal text )
    RepeatString=""
    While (number > 0)
        If number And 1 Then 
            RepeatString = RepeatString & text
        End If 
        number = number \ 2 
        If number > 0 Then
            text = text & text 
        End If 
    Wend
End Function

For code brevity, the accepted answer is good. But the following function is literally 10 times as fast. And it's twice as fast as RepeatString(). It uses a little-known feature of Mid where it fills the remainder of a string buffer with a repeating pattern in one go...

Function Repeat$(ByVal n&, s$)
    Dim r&
    r = Len(s)
    If n < 1 Then Exit Function
    If r = 0 Then Exit Function
    If r = 1 Then Repeat = String$(n, s): Exit Function
    Repeat = Space$(n * r)
    Mid$(Repeat, 1) = s: If n > 1 Then Mid$(Repeat, r + 1) = Repeat
End Function