Google Earth allows you to do quite a lot with respect to checking the size of any area in the world.
This can be helpful to establish rough first-pass approximations of how big a space is and how many people it might be able to hold.
It's free to use too.
CrowdSize is an app you can download that uses Apple Maps to produce an aerial view of any location in the world.
It has pre-established density levels and it's pretty 'point-and-click'.
It costs $2.99.
Excel is a powerful tool. Most people use a tiny proportion of its functionality.
With respect to counting crowd numbers, it can be very handy to monitor fill-rates and increase and decrease in crowd sizes.
Set up a spreadsheet in advance with all the fields you need and then populate in real-time during the event. It can be of immense help.
It's on most computers, in fairness.
There are plenty of crowd-modelling software packages available.
There are VERY FEW events that NEED this level of crowd-modelling. Generally speaking, what you can learn from crowd-modelling software you can usually learn without it if you know what you're doing.
Pedestrian Dynamics is quite a good one, in our experience.
You can get a 1-month free trial.
There are a lot of different tools you can use to count crowds and ascertain the size of a crowd at a given event.
We tend to use Google Earth a lot for our first-pass assessments & approximations.
It's quite useful.
Crowd Counting for Event and Crowd Safety
Crowd Counting with Prof Keith Still | Inside the Box