This number was obtained with a manual count of this google query.
H-numbers can also be computed (approximated) automatically using the H-number'O'matic web interface.
This method is not accurate when the author name overlaps with that of other authors in the field.
The following query, which
corresponds with this Google query results in h-number 16. With the more lenient E. Visser query the H-number'O'matic scripts returns 20.
It turns out that two of the 20 are articles with other Visser co-authors. The restriction of the query above is too strict again.