What does 合痕类的个数 (hé hén lèi de gè shù) mean in this mathematics article?
I am absolutely not a mathematician, but here is my stab:

isotropic: "having the same properties in all directions,"

In your case, a regular matrix, your example showing the vertical and horizontal elements to be the same.

合痕：isotropy
合痕类：isotropic
合痕类的个数： isotropic elements / individual elements
矩阵: matrix

本文继续讨论n=4的情形，
This document goes on to discuss n=4 matrices
给出了(4,k,1)-拉丁阵，(4,2),(4,3)-和
showing (4,k,1) - Latin matrices, (4,2), (4,3)- and
(4,4)-拉丁阵及它们的合痕类的个数。
4 by 4 Latin matrices and their isotropic elements