It really depends on your requirements, both functional and cost. Having a full stack per customer can be great for a lot of reasons. It's probably the safest because you never have to worry about something getting messed up in the code and creating cross-customer access. Once you're sure the environment is set up correctly you can sleep well at night. You also don't have to worry about one customer doing something to impact the performance of other environments (unless you're using shared infra, like putting all your DBs on a single cluster). And it can make maintenance easier, for example you can data migrations can start with small and/or less important customers for practice. It also can give you more flexibility if you need to make special snowflakes (i.e. some big customer wants weird IP whitelisting rules that can't work with your normal env setup).

Downsides are that it's probably more expensive and more work. Even if your infra spin up is totally automated, you still need to keep track of all the environments, you still need to keep your Infrastructure-as-Code (e.g. your terraform scripts) up to date, more can go wrong when you make changes, there's more chance for environments to drift.

So, in short, separate stacks usually means more safety & simpler application architecture in exchange for more cost and more effort to manage the fleet.