More than One One

Question 1

When do you use Uno

Answer 1

When you dont say the noun afterwards or when you want to specify there is only one of something
Uno = 1
Uno chico = 1 boy

Question 2

When do you use Un and Una

Answer 2

When there is a noun (masculine or feminine) afterwards
Un chico = a boy
Unos chicos = a few boys