I'm not certain this will do everything you are hoping for, but perhaps it will help
In[1]:= 2 + x^2 - 4 y + C[1] + C[2] + C[3] /. C[1] + C[2] + C[3] -> C[4]
Out[1]= 2 + x^2 - 4 y + C[4]
In[2]:= 2 + x^2 - 4 y + C[1] + C[2] + C[3] + C[5] + C[8] /. Plus[C[__]] -> C[i]
Out[2]= 2 + x^2 - 4 y + 5 C[i]
where the 5 or the i in 5 C[ i ] has perhaps no great significance.
Be careful to check each time you do something like this. I was actually surprised when it worked on the first try.
