Whilst not a "collector" countries, making a list was interesting. I'm not 100% sure it's complete, but certainly close. I feel old when I'm beginning to forget visits. I count 35 (33 if UK is regarded as one):
Norway (native), Sweden, Denmark, Finland, Iceland, England, Scotland, Ireland, Northern Ireland, Germany, Poland, Czech Republic, Slovakia, Netherlands, France, Austria, Lichtenstein, Switzerland, Slovenia, Italy, Spain, Greece, USA, Canada, Peru, Brazil, Uruguay, Argentina, India, South Korea, Japan, China, Singapore, Malaysia, Indonesia.
Maybe a couple of more if I count countries in which I've just been through airport transit. Portugal, I know.
Most visited countries, at least 5 times: Sweden, Denmark, England, Ireland, Czech Republic, Austria, USA, Japan. 3-4 times: Finland, Germany, Netherlands, France, Spain, Italy, Slovenia, Canada.