Nouns with the same singular and plural form:
* Fish: One fish, two fish.
* Series: One series, two series.
* Species: One species, two species.
* Deer: One deer, two deer.
* Means: One means, two means.
* Corps: One corps, two corps.
* Offspring: One offspring, two offspring.
* Head of cattle: One head of cattle, two head of cattle.
* Aircraft: One aircraft, two aircraft.
* Fruit: One fruit, two fruits (Though "fruits" is more common in the context of different kinds of fruit)
Let me know if you'd like more examples!