HL7 OBR OBX relation

Whats the cardinality between OBR and OBX.
1: N ?
There would be one OBR for 1 or multiple OBX ?
Is there any scenario where message will have more than one OBR ?

Typically 1:N, though it could depend on the message. I don’t think v2 supports tying a single observation to multiple orders, though there are some workflows where that might be possible. There are definitely messages that support multiple OBRs - you’d have to look at the v2 spec to explore the cardinalities.

Note that this forum is focused on HL7 FHIR, not HL7 v2…