jab, you might not like hearing this, and I hope I don’t offend you, but your mother may be exhibiting signs of a more serious problem than you realize.
“Collecting” is a problem seen fairly often in the cat fancy, especially by rescuers. I wish I had some URL’s I could post for you, but I don’t know if anyone has put this together in a formal way. Several rescuers, prominent breeders and/or judges, and therapists involved in Companion Animal Therapy have posted about this to some of the lists I am on.
“Collectors” are usually middle-aged or elderly ladies living alone. Most are suffering from ‘empty nest’ syndrome - their husbands are dead or gone, the children are all adults and out on their own, and they don’t have any really close friends or compelling outside activities.
At first, the cats are sort of ‘replacement children’, and their is really nothing wrong with that as long as it is kept under control. However, “collectors” continuing accumulating cats until they have too many to feed and care for properly. They generally either won’t or cannot afford to have the cats altered, so have a constantly growing population. They begin allowing the cats to take over their house and their lives, and feel powerless to do anything about it, and guilty for thinking of doing so, because the cats are also innocent creatures for whom they have assumed responsibility.
“Collectors” gradually withdraw from contact with family and friends and eventually become virtually hermits, because their house is a mess, no one really wants to come visit because of the cat problem, and the people who do come visit complain about the cats, which the “collector” absolutely refuses to hear. It sometimes appears that the cats are actually an excuse the “collector” uses to drive other people away. In extreme cases, the house becomes virtually unlivable, most of the cats are sick and/or malnutritioned, and someone eventually contacts the local animal control/humane society and reports the situation. Officials then sweep in, confiscate the cats, maybe fine or otherwise cite the "collector’, and leave.
This is rarely successful, because the problem that originally led to the “collecting” behavior still exists and is often made worse by the removal of the cats. Before long, the “collector” will have started accumulating cats again.
Clinical depression seems to be the root cause of this behavior in the cases that have been evaluated. The situation actually becomes a vicious circle that maintains and exacerbates the depression. The “collector” is lonely and feels unwanted/unneeded (and is probably already clinically depressed). She adopts one or two cats as companions to fill her life, usually neighborhood strays. However, there are lots of strays and removing one or two in a specific area seems to create a vacuum that draws in replacements, which will then also be adopted by the “collector”. More cats means more work, and the depressed person often doesn’t have much motivation to constantly maintain cleanliness (“Well, I’ll worry about cleaning that up later - no one’s going to come see me anyway. I’ll take another nap instead.”)
It doesn’t take long for a pretty big mess to accumulate, and when the “collector” sees how much trouble a major clean-up is going to be, she becomes more depressed. She may occasionally go on ‘clean binges’ and get things straightened out, but when the increased number of cats returns things to the same condition in just a few days, she will usually get so depressed that she will give up cleaning entirely. Things really go downhill from this point. 
Please don’t take offense at me for posting this - I am NOT implying that your mother is a “crazy cat lady” or something. What I am suggesting is that she is clinically depressed and on the verge of developing a bad situation. If you think I’m totally full of bullshit, blow me off - you know your own mother and her situation, and I don’t. But if the information I’ve posted here rings any alarm bells, please see about getting her some help.
If you would like, I can ask around the cat fancy for more information. Someone might be able to provide a website with a better profile than what I’ve given you, written by professionals, along with suggestions for how to deal with the problem if you think one exists.