Question about urinary retention --how much is retained and what is problem if it doesn't come out totally?

Varies. The amount of urine retained varies and usually depends on the cause, whether the bladder outlet is obstructed or the bladder squeezes poorly. This can be measured by bladder scan after void. Efficient bladders empty 100%, but in selected cases, 60% emptying can be tolerated. This retained urine can become infected, however, and large volumes can further affect the bladder's ability to expel urine.
Scanning ultrasound. Bladder scan ultrasound can tell urine volume retained without needing to be catheterized.Problems with retained urine include risk of urinary infection, increasing volumes retained as time goes by leading to increasing bladder atony (inability to contract efficiently due to over-stretching of bladder muscle). Development of complete urinary retention & inability to urinate. Consult with urologist.