The numbers in the text refer to the two assignments. I’m not totally sure whether this answers better the question about density or diversity; the phenomenon are interwoven in the answer.
1) My background is economic and social history, but my main interest is in youth and subcultures. While the dominating theoretical view on subcultures is the viewpoint developed by the Birmingham Center for Contemporary Cultural Studies (CCCS). In their approach, subcultures and the associated subcultural style is seen as a solution by the working-class youth to the social pressure of upward mobility facing the British society in the 1960s and 1970s.  Although over 40 years old, the approach is still so widely spread that it remains as a relevant object of criticism.  The main criticism is a construction of a too rigid and narrow concept of subculture that is rooted in social class. 
The influence of CCCS has overshadowed other subcultural theories. Developed contemporary to the CCCS, Claude S. Fishcer suggested “a subcultural theory of urbanism” in 1975. While the CCCS approaches subcultures from the point of view of the collective identity of youth groups, Fischer’s approach is more macro-oriented and is based on the influence of urbanization on social life. According to Fischer “there are independent effects of urban size and density” that – simply by the sheer amount of people – create the possibility for unconventional subcultures to grow.  As the CCCS, Fischer associates deviant practices as a part of subcultures. However, as the subcultural theory has developed, deviance is not anymore seen to be a part of activities associated with young peoples’ social groupings. 
Therefore, Fischer’s approach allows to approach the influence of masses on diversification of social life. Fischer’s theory includes four points that can be summarized as:
- Population concentration causes distinctive populations through intensification of social processes that lead to diverge ways life; including the temptation of central areas for migrants
- The larger the population, the greater the structures that support differentiation and intergroup relations that strengthen the differences
- The larger the population, the bigger the possibility for the adaptation of exponentially differing lifestyles
- The larger the population, the greater the amount of unconventiality
Therefore, the model is a hermeneutic entity where the different principles strengthen each other. 
2) While Fischer discusses the relationship between population size and diversity to a great extent, he does not provide quantitative amounts that could be used in the model to actually somehow “quantify diversity”. However, the keyword of Fishcer is critical mass. The concept is used in other subculture literature. For example, in Nick Crossley’s research on the punk and post-punk worlds in UK reaching critical mass is seen to have multiple different positive effects on the subculture from encouragement to sharing of rehearsal spaces and equipment.  However, as an analytical concept it is practical and vague as it is possibly impossible to pinpoint a number, or ratio of population/diversification. On the other hand, it is practical because it is vague. Therefore, the idea of critical mass behind the diversification of urban life is useful mainly as a theoretical tool.
Bennett, Andy. ‘Speaking of Youth Culture’: A Critical Analysis of Contemporary Youth Cultural Practice. In: Woodman D., Bennett A. (eds) Youth Cultures, Transitions, and Generations. Palgrave Macmillan, London. 2015.
Blackman, Shane. Subculture Theory: An Historical and Contemporary Assessment of the Concept for Understanding Deviance. Deviant Behavior 2014:6.
Crossley, Nick. Networks of sound, style and subversion: The punk and post–punk worlds of Manchester, London, Liverpool and Sheffield, 1975–80. Manchester University Press. 2015.
Fischer, Claude S. Toward a Subcultural Theory of Urbanism. American Journal of Sociology 1975:6.
Hoikkala, Tommi & Suurpää, Leena. Finnish Youth Cultural Research and its Relevance to Youth Policy. Young: Nordic Journal of Youth Research 2005:3.
Muggleton, David & Weinzierl, Ruper. What is ‘Post-subcultural Studies’ Anyway? In: Muggleton, David & Weinzierl, Ruper (eds.): The Post-subcultures Reader. Berg, Oxford. 2003.
 Blackman 2014.
 Hoikkala & Suurpää 2005, 290.
 Muggleton & Weinzierl 2003, 3-5.
 Fischer 1975, 1320.
 Bennett 2015.
 Fischer 1975, 1324–1330.
 Crossley 2015.