Is there a standard for software requirements specification? As far as I know there are only recommendations, and no formal standard for the SRS document. Therefore there won't be a standard for requirement ID's.
What you're describing is called the ID of the requirement. Each requirement should have a human readable name as well that helps humans identify the general purpose of the requirement at a glance. The only requirement for software requirement ID's is that it is unique in the spec that defines it and that it doesn't change after it's assigned.
Apart from being unique in the spec, it may be useful for the ID to be unique across all of an organization's products and for the ID to identify what product or whether the requirement is functional or nonfunctional. That way by looking only at the ID it can help you determine which spec the requirement would be in, and it allows for better requirements traceability, issue tracking, and project management. It's not important for requirement ID's to be strictly sequential. You'll often remove requirements, and you shouldn't try to maintain a number sequence.
IEEE recommendations reference requirement ID's:
An SRS is traceable if the origin of each of its requirements is clear and if it facilitates the referencing of
each requirement in future development or enhancement documentation ... This depends upon each requirement
in the SRS having a unique name or reference number.
All requirements should be uniquely identifiable.
Here is a template that uses ID's. You can find examples in 188.8.131.52