What are the Key Performance Indicators (KPI) for a SaaS Company

The main metrics to look at is:

• User count and growth: How many users do you have, and how is this growing
• Retention: How many users come back to your site
• Activity: This is define by whatever you provide, but basically you want to know if users are actually doing something on your site

Assuming you sell something:

• Conversion rate (very importante!!): How many users go from free to Paying
• Incremental Sales: in SaaS you have recurring revenue, so by how much does this revenue grow over a given period.

Customer satisfaction:

• How many of your customers would recommend you to others?
• How many of your customers have recommended you?
• How many of your customers like you?

Competitiveness:

• How many times do you loose against your competitors in a deal (or do you win?)

Other stuff to have:

• Customer Acquisition Cost: How much do you have to spend (time + Money + other) to get a customer to sign-up and to convert.
• Life Time Value (tough to calculate but critical): what's a user worth during its life time with you (expected / average)
• Incremental Cost per new User (how much does it cost you to add a new user, careful, at the beginning it might be almost 0 but as you scale there are some incremental steps like adding a server, adding an Sys Admin, etc..)

There are others for sure. To answer your question I would say that you could present those numbers based on assumptions if you are in the Business Plan/pitch phase but they will likely to be wrong. Maybe the investor just wants to see if you have a good understanding of your business and what matters..

While forentrepreneurs.com is very usefull, here http://chaotic-flow.com/ you will find a deep analysis about SaaS metrics, their components, explanations and bechmark.