From the testing that I have done, the maximum number of links for any particular leve is 1+3. I generally do my leves in groups of 4-5 and have found that the SP caps after three additional links. Furthermore, depending on the rank of one's group, the number of difficulty stars also cap after a certain number. For example: a group of five player's ranked 27-32 capped out at 3 stars for a rank 30 leve ( This means that SP was not increased with an additional star). However, the same leve did give additional SP for 4 stars when it was tried with a group that was ranked 34-38, but an additional link (beyond 3) did not change the SP. I hope this helps OP.