The Title Sponsors and the Nationality will dictate this. It is recommended that each team have 50% of its riders from the nations of the sponsors and nationality. This is not a requirement, but meeting and passing that 50% will result in bonuses to the team.
The Main Sponsor will contribute 25%, the Secondary Sponsor 15% and the Nationality 10%.
If your sponsors and nationality are the same then all your riders will be from the same nation.
If, for example, your main sponsor it UK, secondary ITA and nationality ESP then 25% must be British, 15% Italian and 10% Spanish.
If you have 50% British, but only 10% Italian and 2% Spanish you do not meet the minimum requirements, but you will get some bonuses as you exceed 1 part.
Thus there are 4 areas of bonuses. 1 for main, 1 for secondary, 1 for nationality and 1 for meeting all 3 others.
Bonuses for Main Sponsor: More Money or Easier Contract Renewals
Bonuses for Secondary Sponsor: More Money or Easier Contract Renewals
Bonuses for Nationality: Better Wildcard chances or More Money
Bonuses for combination: More Money, Better Wildcard chances and ???