Cardinality in modeling (relationship cardinality) refers to your relationship type i.e. 1:1, 1:M, M:M
Cardinality w.r.t. data refers to how many unique values a particular column can have. e.g. a Primary Key has high cardinality because every row has a unique value. A boolean flag has low cardinality because it only has two values.
TBH, I'm not 100% sure which one the OP is talking about here because you need to factor in number of joins and indexes etc, but from the perspective on relationship cardinality, you can think of some of the common differences in modelling for OLTP ("real time" databases) and OLAP (data warehouse etc).
The former would generally have smaller tables and more joins, while the latter would usually have relatively massive tables which inherently could imply fewer joins, but this is often not the case. It just depends on your system and use-case.