How should White play 21 here?
You notice that your opponent has a better board than you, and so consider reducing your blots to only one by playing bar/23 12/11:
This may seem safe, but in order to win at the backgammon you have to take sensible risks and not play ultra conservatively.
After this move you can only escape your checkers on your 23 point by rolling either a 5 or a 3 so it will be hard to improve your broken prime on your side of the board.
If you want to enter on the 23 point, playing bar/23 12/10 looks better:
Now you have more rolls to make your 10 point, but is that the point you really want to make? You would rather make your 7 point than your 10 point.
And you still have your 3 back checkers on the same point, which is inflexible and makes it harder for you to escape your rear most checkers.
You have to you enter on the 24 point to diversify your escaping rolls, so you consider bar/24 12/10:
You have slotted your 10 point, but as previously noted you would rather make your 7 point.
This move also gives your opponent good 6’s from the bar.
This is definitely a candidate move, but you can get the same flexibility of your back checkers and give ourselves a good chance to make your 7 point by playing bar/24 11/9:
This move leads the most blots of any of the candidate moves. It leaves 4 blots around the board. But those blots are not in much danger as your opponent’s 5’s that hit on your 24 point also duplicate your opponent being able to play 23/18 to escape your broken prime. Also, the 2 that hits for your opponent with 11/9* duplicates their 23/21 to make their 21 point anchor.
You now have 3 active builders aimed at your 7 point, the point that you really want to make.
There is a time to minimise blots and play safe, but when you will be trailing by 30 pips after your roll, then that is not the time.