Every such rule has its local variations, but at Southern stations you'll usually have at least 2 people for a 12 coach train - one to attend to the driver's cab or CD/RA buttons (as applicable), and one to see the rest of the platform. In some cases - for example, where you were at East Croydon - you can need many more. This is specified in the dispatch plan (and on a centralised computer system) for every station. For dispatch plans involving platform staff, these are usually created by considering several criteria; not just the dispatch corridor which a driver's CCTV monitors should be able to show, but also the entrances and exits to the station, assisting with wheelchair ramps for shorter dwell times, viewing specific hazards such as large steps or gaps, places where passengers may congregate, you name it really!
It's interesting to note that in some ways the DOO + platform staff dispatch process is much less efficient and more distracting than platform staff + conductors, especially where legacies from previous operation mean some platforms have either a CD or RA indicator and button, not both. To take a typical example, on some platforms at East Croydon, the numbers of available staff do not allow one person to be by the driver's cab for the "CD" [Close Doors"] tip to be given with a baton after the train safety check, and another by the "RA" button to give the signal to the driver via the indicator mounted on the signal post. This means the person giving the "CD" tip has to show their baton to the driver when station work is complete, watch the doors close and perform their train safety check, then walk back to the equipment cabinet, receive the second tip from their colleagues, open it and push the "RA" button, and watch the train leave the platform.
Unfortunately, this has several flaws. From a punctuality and efficiency point of view, walking back from the car stop marker to the cabinet for anything from an 8 to 12 coach formation isn't terribly quick, especially on a crowded platform, and that's once the staff have made their way across to the right position by the driver's cab in the first place, accounting for whatever prior information they may or may not have about the train formation too. Then from a safety point of view, it's also more distracting than having to watch the platform/train interface whilst walking around, rather than maintaining a fixed position. Additionally, and of course this has been subject of much prior discussion on this forum, there is no way of cancelling an RA indication given in error, and it is immediately acted upon; not so with Southern conductors, who do not interact with the CD/RA except during degraded working, and have a few seconds' grace and time to react to platform staff showing a red handsignal.
With conductor dispatch of power-door stock, ie. everything Southern operates, the platform staff can choose and maintain an optimum position for visibility, and the conductor does not usually have to move from their door panel (the choice of which will be specified by route knowledge, in order to maintain visibility of the platform so far as possible) in order to close the doors and give the ready-to-start signal to the driver. You also need fewer platform staff at times, as of course the conductor can place themselves in one of their positions, whilst still maintaining the same checks of the platform and its surroundings.