Ignoring MVNOs, are there are any other countries that have more than 4 nationwide mobile networks? I think the US has (had?) a handful of regional carriers that survived on domestic roaming but everywhere else I've visited typically has 3 networks competing, maybe a 4th smaller network, often a newer startup (e.g. Free in France, Rakuten in Japan, in the 2000's it was Three/Hutchinson taking this role using newly-opened UMTS licenses).
The Netherlands actually explicitly set aside spectrum in the 4G auctions to be auctioned off to new market entrants (besides the established T-Mobile, Vodafone, and KPN), leading to the entry of Tele2 (until then an MVNO) and Ziggo/UPC (until then cable ISPs and MVNOs) into the market.
However, a few years down the line, Tele2 ended up in a merger with T-Mobile (now Odido), and Ziggo/UPC/Vodafone went through mergers to end up as VodafoneZiggo.
US used to have Verizon, Sprint, T-Mobile, Cingular and ATT. There was an active roaming agreement in at least California for T-Mobile and ATT so they shared coverage there, but otherwise pretty much 5 national networks before the ATT T-1000 reformed, and Sprint died.
I've seen some countries where they had three established GSM networks and then someone starts repurposinf their CDMA network for mobile handsets, but it had been built and operating for years for fixed wireless, phone lines for houses without wires.
