It appears that there is a limit of 18 conditionals in a webform. When clicking the + for a new conditional, after 18 have been entered, nothing happens, no message. It's hard to believe that this is by design as 18 is low, bearing in mind that webforms often have many fields, so possibly a bug, and not a limitation? It's possible I may be able to fund a fix as this is a real problem for the user. If there has to be a limit, 50 would seem to be more realistic.