In terms of all time grand slams in singles / doubles / mixed doubles, Margaret Court also holds the record there with 62. I believe that she is the only player in history to win every grand slam in all 3 disciplines at least twice.
The pre-open era vs. open era dividing line doesn't really matter at all in women's tennis. Unlike in men's tennis, there was no separate professional tour, and therefore no amateur vs. professional split, so all players were eligible to compete in all grand slams. However the 'issues' regarding her singles grand slam count are the fact that 11 of her 24 came at the Australian Open which typically had very small draw sizes, in which most of the players competing were Australian, so they were like regional championships at times. None of those 24 grand slams had a 128 player draw size, although as I said earlier, women's tennis didn't properly justify 128 player draw sizes at the slams until the 21st century when proper depth beyond the top 10-20 finally emerged.
Also Wimbledon was clearly the most important grand slam in those days (while in modern times the grand slams are pretty much equal), and she as a serve-volleyer 'only' won it 3 times (the same number of times as the baseliner Evert), compared to Navratilova winning it 9 times and Graf and Serena winning it 7 times. In general though, grand slam counting only became a big deal from the 90s when Sampras closed in on Roy Emerson's record (which previously was considered to be meaningless with Borg not caring about going after it) on the men's side.