I believe that sometimes you won't get the achievements if you are playing offline. It happened to me before for some of my other achievements. Maybe try getting rid of one of your pyromancies and buy another one back?
There are 16 offensive pyromancies (13 if we exclude mist ones). There are then 4 support pyromancies (7 if we include mist ones). Then there's a total of 20 pyromancies, 19 if we exclude the one from the DLC.
They are really good, in general, quite sad with how they nerfed magic in Dark Souls 3