On the official 2015 test DS18 missed:
reading 2 (SI 37)
writing 1 (SI 37)
math 1 (SI 38)
total SI 224
On the official 2016 PSAT10 he missed:
reading 2 (SI 37)
writing 2 (SI 37)
math 2 (SI 37)
total SI 222
Based on this I’d guess you can miss 6 maybe 7 and still make a 221 but it’s only a guess. Just do your best! Every test is different, one year DD16 only missed one math question and lost 50 points on the SAT!
I threw this together quickly so it may not be perfect, but to get a 221 here are 4 scenarios that will work:
If you get a 760 on the reading/writing you can get as low as a 690 math
750 would require a 710 math
740 would require a 730 math
730 would require a 750 math
There are many ways to arrive at those 4 scores.
On Practice PSAT #1 if you have 760 reading/writing score( which meant you had no more than 1 reading wrong and zero writing) you could get as many as 10 math wrong to get the required 690. That would mean 11 total for that scenario.
If you had a 750 or 760 math (up to 3 wrong) you could get wrong either 1 reading/4 writing, 2-3 reading/3 writing, 4 reading/2 writing to get the needed 730. In this scenario you could get 8 or 9 total wrong.
Just as a rough guideline based ONLY on the PSAT #1 scale, 11 would be the most you could get wrong, but no more than 10 can be Math, and no more than 6 English(with no more than 4 reading or 4 writing).