A yogi can multiply himself into 8 or 9 different persons but because his consciousness is one he can feel the same emotions that other expansions are going through. For instance if one expansion is undergoing painful experience, all the other 8 persons will also experience the pain. This is the limitation of the individual souls but Krishna is unlimited and has unlimited number of forms and he can simultaneously be present in different forms and not be affected by the emotions of other forms. 

Kardama muni multiplies himself 9 times to satisfy his wife devahuti as her desires to enjoy sex life increased when Kardama muni took her to a tour around the Universe. Nowadays which man can do such wonderful things? It is also mentioned that the lusty desire of woman is 9 times more than that of a man and so Kardama muni expanded himself 9 times to please his wife. However we cannot imitate Kardama muni because his lifetime was different and he was on a different yuga where he had lot of austerities as well. He never did any enjoyment with his wife outside of the religious principles. 

When we go back home to the spiritual world we have to be very clear about our goal of seeing Krishna and serving Him. Otherwise on our way Home we might get bewildered as we see various planets where there are higher levels of comfort and sensual pleasures. Some planets will have beautiful men and damsels and others will have many yogis who are expert in doing mystical things. When we see them we  should not get disturbed. Because Devahuti was seeing the heavenly realms, he sex desire got agitated and increased. So it's a trap for us when we enter into a very comfortable environment in the material world. Kardama muni had very well organized plan of how much he can enjoy for sometime and how then he has to give up the attachment to sensual pleasures after enjoying with the wife for sometime. He had his austerities which backed him even when he was enjoying the material sense gratification. But what is our position, if we think like enjoying for sometime and come back to devotional service later we might easily get side tracked because we dont have austerities or time like Kardama muni. 

Kardama muni's time is in heavenly realm and our time is very very limited 60-70 earthly years. Chances are there that we might totally get lost if we try to imitate kardama muni. He was enjoying for many many years with his wife and which passed like a second. But after that he planned to stop and then go for liberation. They had many daughters. But they carefully planned to get a son, subsequently. Both husband and wife were alert and then prayed to Lord Vishnu to get a son. Kapila dev, the Lord Himself came to deliver both Devahuti and Kardama muni. Since Devahuti didnt receive knowledge from Kardama muni she was praying to the Lord to get knowledge and Lord Kapila dev satisfied her desires.