I have been keeping 3 cats in a 30sqm condo for 1 year. At first the smell was terrible but now I have it under control. Here are my tips. 1) Change litter every 2 days and use tofu litter which absorbs odor better than clay. 2) Run an air purifier 24 hours and you can get a cheap Chinese brand. 3) Wash the litter box with vinegar once a week. 4) Bathe cats twice a month with deodorizing shampoo. 5) Open windows every morning for ventilation. The most important thing is to clean consistently and never let odors accumulate.