How many audio inputs does your audio interface deliver into Cubase? Meaning, how many audio inputs does Cubase report as being available. This would limit how many individual sound sources you could record at any one time.
You refer to a “mixer”, which really does seem like an external mixer (separate from the computer) and if you then route a stereo (or two mono) signals into the computer, that is about as “individual” as you can record inside Cubase.
Break it down into a little more details (e.g. your audio interface) and it will be easier for people to help out.