A number is simply any number. It is not limited to integers, you may have numbers that have floating points as well.
The following proves Lua categorizes all numbers as numbers.
number number number number
This is an example of operations you can do with numbers.
Here is a common programming issue.
The floating point issue
Lua thinks .1 + .2 doesn't equal .3! Why? This issue has its roots in the IEEE-754 standard for floating decimal point arithmetic. Most programming languages implements this standard (including C in which Lua was written in). Therefore Lua inherits this behavior.