According to orcz.com, the Ancient Shield has a durability of 32, making it the 4th most durable shield in the game, behind the Hylian Shield (800), the Hero's Shield (90), and the Daybreaker (60).
The Ancient Shield also has a Shield Guard rating of 70, which is the second best rating behind the Hylian Shield (90). This rating affects how much durability the shield will lose when it blocks an attack.
According to the zelda-archive Fandom, the reason why the Ancient Shield is effective against Guardians, is because it automatically reflects most laser beam attacks while guarding. With any other shield, you will need to pull off a Perfect Guard to reflect beam attacks. It's still recommended to perform a Perfect Guard with an Ancient Shield, however, as this will protect the shield's durability.