If in doubt, always call your vet or the Animal Poison Line immediately.
Apple seeds (and cores) contain amygdalin, a cyanogenic compound. When chewed or crushed, this releases hydrogen cyanide. A few seeds are unlikely to cause harm but regular feeding with seeds intact is dangerous. Always core apples before giving them to your dog.
If only a few seeds were swallowed whole (not chewed), monitor your dog. If a large amount was chewed, contact your vet.