As Ewan said, you'll need a load balancer. Some of them are known as "reverse proxies".
Examples include Pound, HAProxy.
If your webservices are hosted on Amazon Web Services, you can use the built-in Elastic Load Balancing, which also includes auto-scaling and many more features.
Note that before you add more instances, you need to consider:
- what is the constraining factor: CPU, memory, I/O. This is important in choosing the right servers/instances
- where is the constraint: SSL/TLS decoding, web server, scripts, or database
- if you use a database and the constraint is not on the database, whether you'll be using a single database shared my multiple front ends, or if you'll need to have multiple databases (and in that case, whether you're going for master-slave replication, master-master, etc.).
- if you do writes, and have multiple database instances, how you're going to handle those writes
- likewise, if you store temporary files, or use sessions with locally-stored data, how you're going to handle requests within a single session going to different instances
- whether it may be needed to have sessions "stick" to a specific instance
Some cases are very easy to split among many instances. Others require a lot more planning, good knowledge of how your scripts work, and careful consideration about the consequences.
Also, in some cases, it might be easier, more effective, and cheaper, to add relevant caches in the right places that to try to scale the number of instances.