We use all sorts of numbering conventions at work in P3. To start with always stagger your numbers by at least 5 or 10 so that in the future you can insert new activities that will fit into your numbering convention.
Also, try and use some sort of smart numbering system. Maybe the first digit would represent a project name or number. The next digit could represent a particular part of that project and so on.
Using the activity ID number to solve your problems is very hard. Try to incorporate the use of codes to perform various sorting capabilities. They seem to give you the most flexibility.
my 2 cents from years of experience,