Sure, here are the plural forms of the given single countable nouns:

1. a man → men
2. an architecture → architectures
3. an umbrella → umbrellas
4. an ox → oxen
5. a baby → babies
6. a woman → women
7. a child → children
8. an ant → ants
9. a half → halves
10. a life → lives
11. a key → keys
12. an ink-pot → ink-pots
13. a goose → geese
14. a watch → watches
15. a play → plays
16. a boss → bosses
17. an interesting book → interesting books
18. a good teacher → good teachers
19. an expensive pen → expensive pens
20. an intelligent student → intelligent students