So I'm gonna try to keep track of how many souls and what bonuses from lich. Updates will be sorta random, as I remember to check stats during the day. Please keep in mind that these numbers do not reflect when the stat actually changed, but when I noticed the change. Also my trigger to track the total number of souls is a simple thing and therefore doesn't count properly if more than 1 corpse is present when lich drain is performed, so the numbers are also slightly off because of that.

SoulsStat bonus
139Str: 1, Dex: 1, Con: 1, Int: 1, Wis: 1
218Str: 1, Dex: 1, Con: 2, Int: 2, Wis: 2
910Str: 1, Dex: 1, Con: 2, Int: 2, Wis: 3
1669Str :2, Dex: 2, Con: 3, Int: 4, Wis: 4
2002Str :2, Dex: 2, Con: 3, Int: 4, Wis: 5
2640Str: 2, Dex: 2, Con: 4, Int: 6, Wis: 6
2980Str: 3, Dex: 3, Con: 4, Int: 6, Wis: 7
3212Str: 3, Dex: 3, Con: 4, Int: 7, Wis: 7
3391Str: 3, Dex: 4, Con: 5, Int: 7, Wis: 8
3795Str: 3, Dex: 4, Con: 5, Int: 8, Wis: 9
4576Str: 3, Dex: 4, Con: 6, Int: 9, Wis: 10
5609Str: 4, Dex: 5, Con: 6, Int: 9, Wis: 10
6200Str: 4, Dex: 5, Con: 6, Int: 10, Wis: 11
7289Str: 4, Dex: 5, Con: 6, Int: 10, Wis: 12
7486Str: 4, Dex: 5, Con: 7, Int: 11, Wis: 12