Potion saving for Tier 5 Raids

No easy answer.

Depends on how strong force is, and most importantly moveset.

Blizzard Kyogre might need 15-18 before you won't go through more than 6 people.

Thunder Kyogre might only need as 6-10.

Hydropump I'd put at 8-12.

Those are just ballpark ranges those numbers would be higher or lower for a lot of reasons:
1) Weather. You can add or subtract 1-2 based on weather. Sunny is good for subtracting. Snowy + Blizzard is shudder. Wierdly enough Rainy doesnt make a big difference since it also boosts electrics.

2) Level of Counters, not trainer. Level 40 Counters will survive more and save revives vs level 30 counters.

On another note: Consider a tank or two. Especially vs blizzard. Kyogre with Thunder at level 30 works decently vs HP/Blizzard

Wait for how long? I don't know how anyone is supposed to know the answer to this. Nobody but you knows how much waiting helps with getting more people into your raids.

Wait for how many players? It depends on how good those players are. On average I'd say you want 10+ players if you don't want to use a second lineup. But you could accomplish it with fewer if everyone has a good lineup or the other players are biting the bullet for you and going in with multiple teams of high DPS glass cannons.

Anyone who does more tier 3 solo than Kyogre raids has hundreds of revives. It is an easy fix to your problem.

In Singapore, almost every legendary raid reaches twenty trainers immediately after the egg hatches, but I still need to use Blissey as my anchor (especially against Blizzard).

If you are short revives only, battle until you get off your charge then dodge to get dodge bugged. Use all 6/10/12? this way if possible then potion after the fight. You could also use dust to power your fainted mons after they faint, giving 1-4 HP upon power up revive.

If short potions, may need to fight with 1/2 HP revived attackers only. Would need 2 or 3 bar charge move though to get any sort of damage in.

If short both, spin more and hope RNG is in your favor.

Now to answer your question. Hook up through Facebook or Discord or... to find a group. Then coordinate so that you KNOW the when and where and how many. Simply showing up and hoping doesn’t seem like it would work as well unless it’s NYC or Singapore or some place like that swamped with trainers. Good luck.

I would say wait for twice the number of trainers you need to do a successful raid. Give up if you get to the 7 minute mark, before enough trainers show up.
In my case that would make the one squad number around 12. Adjust up a bit if there are more casuals in the raid group, down a bit if you know everyone has an optimized squad, or the boss has thunder.