Also, the more trade ports that you have in total, the more freighters you will have, and the greater the income that you recieve from trade.
Are you sure about that???
Fairly sure it is based on number of tradeports per planet multiplied by X where X is calculated based on the length of your longest trade chain...
Trade ship numbers only come into it if they are destoyed as this reduces a % of a tradeports income as the ship has been killed and needs to be rebuilt.
But is it equal across all trade ports based on the chain.
Yes (AFAIK) - every trade port gets the same multiplication of X, regardless of where on the map it is. It does not matter WHERE they are as long as one is at each location to maximise the chaining potential.
(Broadcast Centres DO matter as this is where the culture originates)