I guess it would be easy to find out (that wonderful google), but I'm betting Thayer David had more episodes. When I attended my one and only DS Fest in NYC in 2003, John Karlen stated during a cast Q&A that he was never under contract, unlike most performers. He stated (paraphrasing): "Dan and I agreed on things with a handshake and five dollars." He was free to come and go as he pleased and often did so, having whatever his character was written out and he would spend months away from the show to pursue other things. He stated that it wasn't until the last couple years of the series where he stayed on because he enjoyed it along with a constant paycheck. Thayer David also came and went, depending on plot and character, just as most other actors/actresses. Jonathin Frid was given more than a month vacation during the 1897 story (the only stipulation being that he didn't get a suntan); Joan Bennett received quite a few breaks, her character going into a coma or locked up in a hoo-hoo hotel. TLATKLS got her long vacation also during 1897 when she was blown away and eventually came back as Kitty. Louis Edmonds got his breaks, too. The list can go on and on.
Gerard