So, even with "po" Pocitat is imperfective verb. What's the perfective pair of this one?
spočítat or vypočítat
Ještě: "propočítat", "přepočítat se", "napočítat", ...
Shouldn't it be počitájí (with a long á) and not počítají (with a short a)?
It's the same as dělat, dělám, dělají
Could I also use "jejich" instead of svá?
Yes, but svá is so much better here unless you are stressing that it is THEIR pigs they are counting, as a reply to someone who suggested they were counting pigs. And even then svá is perfectly fine even when stressing.