From FIXwiki

FIX Data Type NumInGroup

Name	BaseType	First introduced	Deprecated
NumInGroup	int	FIX.4.3

int field representing the number of entries in a repeating group. Value must be positive.

Notes

Quoting from http://fixprotocol.org/discuss/read/af6bffce...

As a recipient of messages you should be defensive, i.e. be prepared for NoXXX=0. From a FIX compliance point, NoXXX should not be sent but is technically permitted if the repeating group is optional. The ambiguity was clarified in FIX 5.0 SP2 Volume 1 as follows:

- Sending NoXXX=0 (e.g. NoPartyIDs=0) for optional repeating group is valid but not recommended.

- Recipients should be able to accept NoXXX=0, but Recipients should not require this.

- Senders should never send NoXXX=0.

- For repeating groups that are marked as required, sending NoXXX=0 is not FIX compliant.