See: https://www.shihtzu.se/ak/avelsregler/

Excerpt... The male dog should have a maximum of six offspring before the age of 2. However, the number of offspring can be more if the male dog has mated two different bitches. SST recommends a maximum of two litters per year per male dog.  

The total number of offspring from the same male should not exceed 5% of the total number of puppies over a period of four years. This means about 8 litters during the male's lifetime if the average per litter amounts to 5 puppies.  

SKK recommends a maximum of 6.25% in inbreeding coefficient. Above that limit is not considered a safe pairing.  

To get a wider breeding base, SST recommends an inbreeding coefficient of less than 2.5%.