ClearAll[Mertens]
Mertens[n_] := Total[MoebiusMu[Range[n]]]
Grid[Partition[Mertens /@ Range[99], UpTo[10]]]
Count[Mertens /@ Range[1000], 0]
SequenceCount[Mertens /@ Range[1000], {Except[0], 0}]