that depends on whether you want to work toward pro recordings or hobby stuff, certainly if you wanna go pro I would suggest getting a sound card or audio interface, and it would be better for you to use software made by the same manufacturer to guarantee multi tracks.
then you need a mixer for obvious reasons "sheesh this gets expensive".
however in theory you can carry on using software only.
I assume you use cubase or something similar? and that has a multitrack mixer built in.
the individual tracks on the main arrange page is where you edit all the audio.
the reason for getting a sound card/interface is to control seperate channels externaly and get better quality recordings and playback and to add external effects to each channel via hardware units through a mixer.
please tell us how you make your tracks and we might be able to be more specific in giving advice.