20 mmol is around 320 mg/dl and some symptoms would be:
Excessive thirst
Frequent urination
Unexplained weight loss
Glucose in the Urine
Extreme hunger
Tiredness or fatigue
Rapid changes in vision
Numbness or tingling in the extremities
Slow-healing sores or infections

Some potential complications (If this person does not get their sugar in check) could be:
Loss of eyesight
kidney problems
liver problems
urinary tract infections
poor circulation
high blood pressure
high cholesterol
heart disease
stroke
heart attack
vision problems
sores that don’t heal
risk of infection, possible amputation

It needs to be lower than 20. 20 won’t kill you right away, but it will certainly kill you slowly. This person should be shooting for numbers from 4.5 mmol/l to around 7 mmol/l.

Edit: Wanted to add, if the person in question is a T1 and produces no insulin of his or her own, and if this is a one-time thing, or a very rare occurrence, it isn’t "go to the hospital right away" bad. Numbers this high can cause serious problems over time, though, and should definitely be lower than this.

If you mean 20 mmol, it is very high. Unfortuately with diabetes, you can always not have any symptoms.
That is why testing is so important.

Go to an emergency room if
You have nasua or vomiting
Fruity smell on breath
Confusion
Labored breathing

Blood sugar that high need medical attention if you can not get it under control within a couple of hours.

If you don’t know exactly what to do, at this blood sugar level call the doctor and ask.

When blood sugar is over 11 mmol (200 mg/dl), test for ketones.
Do not exercise, this can raise blood sugar even more do to lack of insulin.
Drink a lot of water.
Check blood sugar every 30 minutes.
Take more insulin, but be aware of how much insulin is already active, and watch for low blood sugar for the next 4 hours. Stacking insulin (taking more insulin when insulin is still active in the body) can cause extremely low blood sugar.

Symptoms can be
High blood pressure.
Fatigue.

Taking insulin can be quite complicated.
You have to always track how many carbohydrates have been consumed.
You nede to estimate how much insulin is still active.

Different types of insulin have different times to onset, peak absorbsion and duration. This also varies per individual.

Novolog which is a fast acting insulin, will still be active in the body after 4 hours. The peak effect is at about 90 minutes, so after 90 minutes it has mostly been absorbed.

However, ONE high blood sugar reading seldom produces ANY symptoms. in order to produce symptoms, the reading must be high for a LONG time, like several days. if the person is taking diabetic medications (like insulin injections) The blood sugar does not stay that high for very long.

if you are using American Readings, then this Value is WAY TOO LOW. this person may be passed out, bu definitely acting ‘;drunk" – confused, dizzy, maybe nauseous. you need to call 911 NOW and have this person transported to the hospital.

Since it is clear that YOU do not know what these readings mean, you need to be on the safe side and take this person to the emergency room.