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e.g. Energy Grid Assemblies - the total galactic supply at the moment is 160,000 - less than half of what is required to repair a station.
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The total available doesn't drop by the amounts taken from the market though. It takes some time for the amounts to drop. Plus small amounts are usually generated to keep the amount above zero.
The important number isn't the total amount of a specific commodity, the important number is the difference between "amount created every 10 minutes" and "amount taken from the market in that time".
If I remember correctly the stations produce new commodities every 10 minutes.
In short: If the stations create more of the commodity than players take away, everything is fine.
If the supply of the required commodity drops to 0 it might be a good idea to switch to a small or medium ship to grab the steady tickle that station produces instead of doing long range trips.
It all depends on how long the trip to the next station with enough supply to fill a big ship takes.
Example: Station A produces 100 units every 10 minutes. The next station is 20 minutes away. Any ship transporting less than 400 units will be ineffective on the long range route.
Getting that balance between transported cargo vs. time right can be difficult and is influenced by a lot of things.