This is a case where you've got to balance health needs with quality of life. Yes, it's important to be careful about infection, so if visitors have colds or coughs,
you should definitely have them stay away. But in general, we encourage patients and caregivers not to let fear of infection drive them to live in a bubble. Talk to your doctor about how low your father's count is: Unless the doctor specifically recommends isolation, it's probably just a case of avoiding exposure when possible.
Because quality of life is so important, you have to take that into account when balancing the benefits and risks. Having visitors over or getting out for a walk or lunch can be an important way to lift your father's spirits and keep him from becoming depressed, so that's a worthwhile trade-off.
Think of it as a calculated risk. If you head out to the mall, maybe there will be people there who are carrying germs. But your father gets to walk around, see people, exercise, and take his mind off his illness. You could go for just half an hour and the amount of exposure wouldn't be that great, and meanwhile your dad's staying active and involved. You want to consider which parts of life will allow you to maintain some sense of normalcy. Staying home may just make your dad feel like more of an invalid.