How many DB instances to use? When to switch to container services?
The choice of the approach is a risk assessment, if one of the DB client (application) goes wild and bring your server instance down, what is the cost in term of human hours lost during the outage ?
If you run the DB yourself instead of managed you have to include your time costs to update it, backup it, monitoring it, etc. is it really cheaper ?
All this is very company specific and need to take into account the human cost, does a staging environment down block 1 or 80 devs ? Are the devs junior or senior ? The cost per hour of outage change a lot from these simple two metrics.
At the end of the day, that's a judgement on accepted risk of outage and capacity to restart versus administration time to avoid the outage for which there's no magical mathematics formula.