As of today there are none.

In Gen 1 (red/blue/yellow) there were four: flying, rock, ice and ghost.  
In Gen 2 (gold/silver) we got the first pure ghost in Misdreavus and the first pure rock in Sudowoodo. But the newly introduced steel type wasn't used alone.  
Gen 3 (ruby/saphire) introduced two pure steel types, Mawile and Registeel, as well as three pure ice types in Snorunt, Glalie and Regiice.  
Gen 5 (black/white) had the legendary Tornadus, which is pure flying.  
Then in Gen 8 (sword/shield) the first nonlegendary pure flying type got intoduced (Rookidee and it's evolution Corvisquire).

Gen 4 has the promo legendary Arceus and Gen 7 the legendary Silvally both of which can be every single type depending on the carried item. Both are normal type when not carrying an item.

The dark and fairy types had pure specimens from their inception.