number not in order and having gap between number

OK. set it to no Gap. 

No gap will get the sequence and unused number from system.

Like, if you make a quotation as Quotation 001, then second Quotation number will be 002. After delete this second quotation and create a third one, it will be:

Quotation 002 with "no gap"
Quotation 003 with "Standard"

The "No gap" will get the newest number in use

The standard will keep incrementing no matter what happens to the previous ones.