There are no definitive answers to your question. In my personal testing, I found a range of about 500 feet through one exterior wall using 5 GHz. However, this will vary depending on the client (iPad vs. MacBook etc). Its range will be a function of your wireless environment including the number and type of intervening obstructions and the presence or absence of competing wireless networks.
The published maximum number of simultaneous users is 50, but as I understand it this is more of a guideline than an absoute limit. For example, if everyone is streaming bandwidth-demanding content such as video at the same time, their wireless performance will suffer before you reach 50 users. If on the other hand people are simply connected to the Extreme without doing anything particularly demanding, there can be more than 50 users on the network.
Last question first, since that is the easy one......approximately 50 (fifty) users for wireless depending on how active each user might be.
The first question is much more difficult, as distance is not really the issue with wireless.....obstructions are the issue.
A wireless signal can easily travel 300 feet or more with no obstructions in the signal path.
But, a typical wall of sheetrock or wallboard in a home will absorb....on average....15-20% of the wireless signal. Brick or adobe will obviously absorb much more. A ceiling....because it is thicker....will absorb about 25-35% of the signal.
So, if the wireless signal must pass through several walls and a ceiling, there won't be much, if any, of the signal left....even though the distance that the signal travels is not great...for example, 20-25 feet.
Other wireless networks around you at the neighbors can also affect the relative strength of the wireless signal.
The bottom line....locate the AirPort Extreme where wireless devices will receive a signal with the least amount of obstructions in the signal path. Your actual performance cannot be predicted in advance. You will not know how well the AirPort Extreme works for you until you try it out in your home.
If you find that the signal does not reach as far as needed, you will need to add additional "extenders", like an AirPort Express to provide more signal coverage.