If install several lights do I need to add them one by one to Homey? Or if I put them all in Pairing mode will they be able to be added all at the same time?
Which protocol? If you mean connecting via ZigBee or Z-Wave, then you need to add each one individually. If you are linking them via LAN/Cloud, they can usually be added all at once.
Zigbee devices
Zigbee devices have to paired one at a time with Homey.
So if I have 5 bulbs in a light fitting then I would have to add a bulb and then pair it. Then remove it and add another, and then continue until all added
Or if I try to out in pairing mode I assume that the currently paired bulbs will look to be re-paired
Yes.
With fittings it’s typically not possible to put a single bulb in pairing mode as the fitting will power all bulbs at the same time. So if you need to put a bulb in pairing mode by powering it on/off a certain number of times, all bulbs in the fitting will be put into pairing mode. So you will need to pair the bulbs one at a time.