two
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Digits or decimal digits more specifically. The decimal system has 10 unique digits 0-9.
It is -987. The smallest positive 3-digit integer with unique digits is 102.
Four.
Started in ancient Rome, this unique numbering system is widely used in many countries in the present time. Then in the the 9th Century America started to use roman numerals.
It will be asumed that you are refering to the binary number system, which is used in computers and digital devices. A binary number is always either 0 or 1. This is also commonly refered to as "low" or "high". This is in contrast to the decimal number system, where the numbers can vary between 0,1,2,3,4,5,6,7,8 or 9.