In my case, I have Yeva’s Forcemage 10 times.
I’ve got several common cards at 20+
Most Rare copies are 6 Jace Sanctums
Also have 4 hangerback walkers and infinite obliterations for Mythics
I’ve got 11 Mantle of Webs, 10 of something else, and a few with 9. I’m sure anyone who has close to a complete collection will have closer to 20+ of some commons, as above.
I’m sitting on 33 Commons with 10+ duplicates, the winner being Firefiend Elemental at 15. I don’t think I’ve ever even played the card. There’s a half dozen sitting at 14. On the Rare side, I’ve seven Displacement Waves. Woo.
Common: 14 veteran’s sidearm
Uncommon: 4 possessed skaab, 4 necromantic summons, 4 war oracle
Rare: 2 displacement wave, 2 orbs of warding, 2 hallowed moonlight
Not a single mythic card
I’ve got 31 of these behemoths…
Looking at rares, I don’t have that many - but I have 4 of the Pia and Kiran. That’s the only rare card I win on red PvPs for some reason. 
Ouch, now that’s bad
31? Jeez…I’m currently at 192 cards out of whatever the current max is.
Commons, I’m almost in double digits, but currently cap out at 9 Titan’s Strength and Caustic Caterpillars.
Uncommons, 5 Reclusive Artificers (proving to be neither reclusive nor uncommon) and Anchor to the Aether.
Rares, 3 Herald of the Pantheons.
Mythics, 2 Starfield of Nyx (aka double sadness).
Card Inventory: 222
25x Maritime Guard
Eleven murder investigations.
Apparently, D3 go! thinks I live in Chicago…
16 Evolving Wilds for me.
IIRC, one of the first versions of the game would tell you in the inventory screen how many unique cards you had, and how many you had total. For example, 133 unique (256 total). I checked the inventory screen and it looks like this feature has been removed, so it go me curious: how many dupes do I have now?
I have 225 unique cards - I’m only missing a few rares and most mythics. I have 954 duplicates in my inventory. Almost a thousand cards - nearly a dozen big boxes worth, junk. There was talk a while ago about future developments offering something to do with the dupes. All I can ask is a heartfelt plea: whatever is implemented, please don’t give it a flashy thing that will make it take a year to off-load all these cards 
220 Unique for me missing one uncommon
21x Titan’s Strength and Reave soul
12x Blazing Hellhound
5x Embermaw Hellion
2x Day’s Undoing
i really need to get a Demolish… being oppening packs since december and i still cant get one