On galaxy wide demand, two points. First resources can be sold without a trade agreement, it merely adds a bonus tariff on top of the usual transaction. Secondly galactic stock versus demand is used to calculate the resource price even if no inter empire trade is taking place. If the price of steel rises (through galactic demand) not only the build cost but also the maintenance cost for all ships/bases which use it rises. These costs can triple from low price to high price... Short story it still matters to you.
On your other question about how many bases you need for each resource there is no clear answer. I will do my best. Experience is probably the best gauge here, although that does not help new players. Keep an eye on the stock levels of your starting spaceport: you are given generous amounts to start the game with. As these go down it indicates a need for that resource. The ones which are dropping most quickly are the ones you need to prioritise building bases for. There can be be quite a lag between realising this need and getting a flow of supplies from a base that has to be constructed. To start with early in the game you should aim to get at least one source for each basic resource. Some you will need more of, for example, steel, caslon. If you look around in the design details screen (where you design ships/bases) there is a button which gives a breakdown/summary of the resources used to contruct that ship/base. Check out the ones you are building. If you have demand for a resource and no source if your private sector can find a tradeable source (another empire/independent) they will buy it from them. This can be both good and bad. Also it is not simply a matter of number of bases since they have an extraction rate (number of extractors x tech modifier) and mining locations have a % richness of deposit. Look for high % deposits close to your spaceports. Remember colonies also produce resources. Don't worry about the maintenance fee for private mining bases (i.e. anything which is designated 'mining base') since this is carried by the private sector and it is very hard to make them broke. Early game definitely build bases flat out - it is a matter of prioritising for what. I keep going like this all game but some differ on mid/late game strategies.
[Edit: where there are multiple resources of the same type (basis/gas/luxury) present they are all extracted at once, so 50% steel, 20% gold, 10% lead means 80% extraction of 'stuff'.]
I hope that helps somewhat and is not too confusing.
< Message edited by feelotraveller -- 7/21/2012 6:33:52 AM >