Hi JaggidEdje
I haven't actually seen anything written with a set number of graves per crypt but I have been going by lilsimsie's video that mentioned a number that I stumbled across. It's here i built a CEMETERY with The Sims 4: Life & Death if you want to watch the whole thing, but the number mentioned at 4:59 of the video is "they only put twenty graves per crypt", which I presume was provided to her by EA since she was a member of the creator network at that point. It's questions like this that make me miss Prima guides of old. :)
Welcome back to playing TS4! You helped me quite a bit on the Steam PZ forums while I was also on hiatus from the game. I'm swskydancer on those forums but haven't been active there in well over a year for personal reasons. It's nice to be able to answer a question for you, even if I can't be 100% positive it is entirely accurate. It's the only answer I've found so far to a question I had myself.
Since I hadn't played TS4 in three years when I returned, I surely do get why you are a little overwhelmed getting back into the swing of things. I'm playing catch up too. I've only been back for about six months myself and the game changed a lot during my three-year absence.