That depends on many parameters, but generally, here are more details (without knowing your organization, services, etc.).
First of all, see how many people you need on your 1st line. Then try to avoid to put your best people (usually most expensive and most usable on other, more complex places) if your 1st line is doing simple activities (opposite view if your support is complex in nature). Check also if you can afford to have a lot of resources dedicated to the services (e.g. few maintenance contracts). If you have a lot of issues with unknown root cause (Problem Management) - most probably you'll need more resources on 2nd level.