The maximum number of connections that can exist at once depends on 2 different factors:
1. The maximum number of open file descriptors that is supported by the OS
2. The value setting in the VDS global parameters.
The maximun number of simultaneous connections will be limited by the lowest of these two parameters. So it is possible to increase the number in VDS, but you also have to ensure that the system has enough resources to handle the increase.
In regards to your second question about monitoring the number of connections, I believe this could be accomplished by adding a plugin to your configuration. For example, there is an Access Log plugin and an LDAP Statistics Generator. The best solution will depend on what you're looking for. We may also be able to provide some kind of custom soluton to meet your needs.