Honestly these days it really feels like there's almost 2 Interisti for every Milanista in the city, at least 50% more than those guys. And by the city I mean the municipality of Milano, not the metro area which includes places like Monza, Lecco and Novara. In those places, if you're a Milanista, you hate Inter and kinda like Juventus on top. Which is also why lots of Interisti end up hating Milan. Not only their arrogance, but their sympathy towards Juventus just makes them vile.
Entire metro area you probably have a 50-50 split. If you consider the Lombardia region, they may be more than us.
The joke in Italy these days is that there are more Juventus fans in Milano than both Inter and Milan fans combined
For Milan fans, I guarantee that it's probably true - if they're not on a similar level - but whenever you go to Milano it's almost a rule that the largest fanbase you'll encounter is Inter, no matter how big your sample size is.
In fact, many surveys show that the Milanese clubs have barely over 50% of the fanbase of Lombardia. So you can imagine that it's not inconceivable that together they do not have over 50% of the metropolitan area since most of the provincial areas have significant numbers of both Inter and Milan fans and not really that many support their local team above a Milano team.
There are lots of Napoli fans as well in the city. Juventus fans are either domestic immigrants or 2nd/3rd generation immigrants, whereas Napoli fans are mostly current immigrants.
I gather that you were growing up in the late 80s-early 90s period which was a period where Inter fans weren't really vocal and Milanisti were on top of the moon, after being humiliated for over a decade by Interisti for being a Serie B side etc.
They also had an exaggerated attendance which was sparked by the Berlusconi factor. Not only the PR stuff and people wanting to suck up to him, he was also giving out tickets because he wanted to suck up to others. Either way, attendance shouldn't point out that much as the fanbases are rather similar within a reasonable vicinity of the stadium.
I guess it also has to do with one's social & work circle. When I was interviewing people for the Milano office I was extremely annoyed by how many gobbi came over and back then I was under the impression that Inter & Milan fans were like 90% of the city's population. After that I got grounded to reality and whenever I brought this up, most people would agree on this anecdotal statistic. I met more tourists, immigrants & visitors who supported Milan than actual locals over the last 20 years that I've been going quite frequently for multiple purposes. And most of that was during our banter era, so it's not like our fans were more vocal or anything.
People working in schools are usually a good indicator as they can tell you what teams the kids are supporting.
The gist of it is:
- Milan fans tend to say the city is split between the two sets of fans. That's probably untrue, unless you count the areas that aren't really Milano (ie metro area).
- Juventus fans tend to say they're more than both combined, or at least that they're the most supported club. While farfetched, it's not necessarily false.
- Inter fans tend to say we're the vast majority in the city. I reckon we are the club with the most numerous fans, but we're not the vast majority. Not with all those domestic migrants who are almost exclusively Juventus and Napoli fans.
If I had to guesstimate, I'd say 30-35% are Inter fans, some 25% are Juventus fans, Milan fans are something like 20-25%, some 10% probably are Napoli fans by now and the remaining ~15% is split between various clubs, like Roma, Atalanta, Brescia, Fiorentina, Palermo, Catania etc.
But I trust that these numbers would look much different if it's based on the metro area or in Lombardia. We probably end up 2nd or 3rd there, but my experience is almost exclusive to the city itself.