The throwing team sets the relevant number
Law 18 lets the throwing side determine the maximum lineout numbers. The non-throwing team can have fewer in the stated ordinary setup, but not more, subject to the law’s immediate-throw qualification. The receiver is a separately defined role.
Imagine the throwing team placing four players in the lineout while other forwards stand elsewhere. Seeing four does not mean the team has forgotten to bring its full pack. It may be a deliberately short arrangement.
Count positions rather than familiar shirt numbers
A wide camera showing everyone near touch can make the receiver or another required position look like another person in the lineout. Identify the two lines first, then count their actual participants. Do not count every forward in the picture as part of the same number.
The useful explanation is structural: a team can use fewer players at the throw while retaining others for the next passage. This is an observation about arrangement, not a claim about which tactic will succeed. When an official corrects numbers, the throwing side’s setup provides the reference. A close-up of the jumper alone rarely shows enough to reconstruct that comparison.
Questions answered
Must every lineout contain the full pack?
No.
Should every player near the touchline count as a lineout participant?
No. Identify the defined positions first.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.