I know where you are coming from.
My friend decided to get another kitten as a friend to her young cat and we knew of a cute little fluffy thing that was a barn cat that was a sweetheart - she took it in but it ended up costing a ton in vet bills because the farm didn't deworm, vaccinate or spay / neuter.
It had an upper respiratory infection and all sorts of parasites and was very malnourished. It is finally healthy and the sweetest little cat ever but it had a wild sibling that I felt awful for - I knew it would probably die because of all the health problems and I desperately wanted to help it.
After thinking about it, I realized taking this one kitten in (when I really didn't want a third cat) wasn't really helping anything. Other kittens would be born just as is need of a good loving home. Instead, I decided to foster with a local shelter - it seemed the best way for me to help as many as possible. It isn't feasible for everyone but for me it was the best option.
I'm sure everyone on here wants to help as many cats / kittens as possible. There are lots of ways to help - adopting from shelters, feeding feral cats, TNR, volunteering, donating supplies or money to good organizations... Think about your situation and what you can do - maybe volunteering or donating supplies to a local shelter would be the best for you. Maybe you can take in the little kitten (but if you can take one more in, could you maybe be a foster home and help many cats find forever homes?). Maybe you can help your neighbor with the ferals and try to start spaying / neutering some of them?
In my situation, I figured taking in one poor kitten only helped that one kitten. Because I have the means, space and time I decided to foster so hopefully I can help lots of cats / kittens. Good luck - your heart is in the right place, I'm sure whatever you end up doing it will help.