Uno alone or in a compound number drops its -o before a masculine noun. Un or una is used depending on the gender of the noun uno modifies. It's number one. They stole one truck. They also stole one hand bag.
Cardinal numbers are invariable.
They agree in gender with the noun they modify. This book has two hundred pages. We'll buy three hundred cards. Cien and mil do NOT require any articles, unlike English. I deposit one hundred dollars.