10000000000
10000000000
1000000000
1010x1.65 10x10x10x10x10x10x10x10x10x10=10 billionx1.65=16500000000
The counting principle works like this.Multiply the number of choices you have for each character.If you can have 0000000000 then you would multiply 10x10x10x10x10x10x10x10x10x10 or 10,000,000,000.10 billion.If you eliminate that number that there is one less.
First position can be any one of 10 numbers. One number has been taken so the second position can be any one of 9 numbers. Third position can be any one of 8 numbers. 7,6,5,4,3,2... Last position can only be 1 number as all the others will have been taken. Which leaves you with 10x9x8x7x6x5x4x3x2x1 or 10! (factorial). 10! = 3,628,800 combinations. If numbers can be repeated this makes things easier as you can just multiply by 10 for each position - 10x10x10x10x10x10x10x10x10x10 or 10^10. 10^10 = 10,000,000,000 combinations.
It depends on the capacity of the networks server. In the UK we recently had an overhaul of the dialling codes system, as they were running out of codes. With the introduction of better computer systems, BT was able to increase the capacity of the network (using the new codes) to provide enough capacity for at least the next 20 years.