This is the 167th most frequent French word.
nombreux
Numerous.
Here, 'nombreux' is used as an adjective describing the plural noun 'invités', meaning 'many numerous' in the sense of a large amount.
Il y avait de nombreux invités à la fête.
There were numerous guests at the party.
'Nombreux' is functioning as an adjective describing the noun 'problèmes', emphasizing the plurality of issues in relation to the context described.
Des problèmes nombreux surgissent dans ce projet.
Numerous issues arise in this project.
The term 'nombreuses' is the feminine plural form of 'nombreux', adapting to agree with the gender and plurality of the noun 'conférences' it describes.