At least 18 out of all the 32 Gunns or attributes should match to make the marriage work.

(2, 12) or (12, 2), this pair brings out the Dwi Dwadash Bhakoot dosh. This dosh creates a lot of financial problems after marriage. Also, could be related to career downfall or loss in business.

(6, 8) or (8, 6), if both the moon signs are in either of the pairs, then it creates a possible Shadashtak Bhakoot dosh. It could lead to incompatibility and severe health problems. 

(5, 9) or (9, 5) distance pair is responsible for the NavPancham Bhakoot dosh. This dosha is responsible for problems like frequent miscarriages, the birth of a defective child or no child.