Adding multiplayer functionality to Cities Skylines could significantly enhance the gaming experience for 2 main reasons 

First, there is collaborative city building. Multiplayer would allow players to collaborate in building cities together. This could lead to interesting dynamics where each player specializes in different aspects of city management, such as infrastructure, zoning, or public services, creating more diverse and efficient cities.

It would also make the game very replayable, which makes the game more fun. Adding multiplayer can significantly extend the replayability of the game. Playing with friends or other players online adds variety and unpredictability to each game session, as interactions between players can lead to different outcomes and experiences each time. 

Determining the exact percentage of players who want multiplayer in Cities Skylines 2 would require conducting surveys, market research, or analyzing player feedback from forums, social media platforms, and other community channels.

However, it's worth noting that multiplayer functionality is often a highly requested feature in many games, particularly in simulation and city-building genres. Many players enjoy the social aspect of gaming and find multiplayer modes to be an exciting way to interact with friends or other players online. Additionally, multiplayer can enhance the longevity of a game, attracting a broader audience and potentially increasing player engagement over time.

To get an accurate percentage, it would be necessary for the developers to conduct thorough market research or surveys among their player base to gauge interest in multiplayer functionality and understand the preferences and expectations of their audience.