It depends. Its not always just about the ratio of chargebacks but also volume.
So you might trigger some flags with the card associations if you say have the following
1) 1% chargeback ratio 2) 1% of revenue result in chargeback 3) Have like 50-100 (forget exact number) of chargebacks a month for like x consecutive months.
If you don't fix the problem, then bad things happen like additional fines....etc.
So for merchants with very low volume, a higher chargeback rate is sometimes permissible. Acquiring banks have their own risk assessment so they may allow it or may not.