How do I get column A and B together in column C in Numbers?

Try the TEXTJOIN function, which is (relatively) new to Numbers.

In C2, filled down:

=TEXTJOIN(", ",1,A2:B2)

If your region uses , as the decimal separator the formula would be:

=TEXTJOIN(", ";1;A2:B2)

More on TEXTJOIN here:

TEXTJOIN - Apple Support

The 1 or TRUE parameter tells Numbers to ignore blank cells, as I think you want here.

SG

Concatenate(a2," ",b2) wil work but will put a space in front of 2nd column contents if the first is empty. You can easily fix this with an if(..) statement if that's a problem.