The only way to earn Development Points in Manor Lords is by upgrading the level of your settlement. You can currently level up your settlement six times, going from a 'Settler's Camp' all the way up to a 'Large Town', which means that you are going to be limited to six Development Points per region. Each region you claim will have its own development tree, so the bonuses from your main settlement will not carry over to your new lands. There is also no way to respec your development tree, so every choice you make is going to be final unless you reload from an earlier save point.

Here are all of the different settlement levels, as well as their requirements, so you can see how much you need to invest for each Development Point in Manor Lords: