You don't want to have a separate webapp for each client because if your application is mostly database oriented, the load on your webapp server is going to be much lesser than the load on the database server. The cost of the webapp server, and more importantly the labor cost of maintaining the server will be a big waste
This kind of model where you want to support multiple clients in your SaaS app is called multi tenancy. There are various multi tenancy models.
This whitepaper does a good job of covering the various multi tenancy models