As usual, it depends. If you have a few large customers and a large number of smaller ones I'd isolate the large ones in their own DBs and put the rest in a single DB. If any of the smaller ones become dominant then you move them out as well.
If all your customers are individuals or small accounts then I'd put them all in a single DB, but I'd still build in the option to redirect to another DB if the software has applicability to enterprise level customers.
Note that it is perfectly OK to upsell the capability to isolate your customers data from each other, segregation options could be offered at multiple levels (all in one bucket, different DB, different cluster), each with their own pricepoint. Some customers will simply demand this and move elsewhere if you don't offer it (and they are typically quite insensitive to price as long as the right boxes get checked for their auditors).
If all your customers are individuals or small accounts then I'd put them all in a single DB, but I'd still build in the option to redirect to another DB if the software has applicability to enterprise level customers.
Note that it is perfectly OK to upsell the capability to isolate your customers data from each other, segregation options could be offered at multiple levels (all in one bucket, different DB, different cluster), each with their own pricepoint. Some customers will simply demand this and move elsewhere if you don't offer it (and they are typically quite insensitive to price as long as the right boxes get checked for their auditors).