Can one AS unite several ISPs, can/does an ISP own several ASes? No matter what I read nobody really clarifies this relationship or at least doesn't writes it down. Or is ISP and AS just a synonym? Is there a special reference in the rfc towards this topic?
An Internet Service Provider is a business entity. An Autonomous System is a construct of the BGP protocol. An ISP can own several AS's or only one.
An autonomous system is
a set of routers under a single technical administration [RFC 1930]
So it's unlikely (but not impossible) for two or more companies to share an AS.
An Autonomous System is a set of routing prefixes advertised via BGP.
While that can match an ISP, providers can also advertise multiple ASes - e.g. from mergers, acquisitions or from customers. On the other hands, customers can have multiple ISPs advertise their AS for redundancy, route optimization, or anycasting.
If your organization is having more than two ISP connect ed to network for internet access and you wants to confiagure EBGP routing protocols then AS number comes into picture
AS number mean autonomous systems number organization should purchase one AS number if not you need to buy from concern entity and isp will have one AS number. In EBGP routing protocols will communicate different AS numbers..
In EBGP configuration different AS number is mapped with each othe to enable communication from isp to organisation..