It is really hard to determine how many plants have gone extinct. The first reason for this is that we can’t say something has gone extinct until about 30 years has passed without finding one.
Experts guess that in the last 500 years there have been 800 species of animal and plant that went extinct. This means that about 160 have gone extinct in the last 100 years. This is out of 150 million species we know about! So, about a tenth of a millionth of the species went extinct. There are a lot more animal species on earth than plant species, so I’d guess that maybe about 20 plant species might have gone extinct in the last 100 years. But that is a total guess!!