18 billion is the max for me, but some try and squeak out 20 billion people.
Are you playing Dark Avatar or Twilight of the Arnor, in TA once you get 20,000 BC in the bank, your income will plummet, so spend such that you don't cap.
If you get a yellow food tile (300% bonus to food) DONT put a farm there
Research the +morale techs
Build Trade Goods or other special buildings that give a CIV-WIDE bonus to morale. Harmony Crystals, Spices, etc.
Find MORALE RESOURCES and ECONOMIC RESOURCES (Little floating crystals on the game map yellow and green respectively) and UPGRADE THESE
So many new players fail to realize that you can send more constructors to these to increase the civ-wide resource extraction (there is a limit of course, for a total of 6 I believe, but you need certain techs to unlock most)
Those are tips for morale and economy. Thinking about morale and taxes as two sides of the same coin is a good thing to do, since afterall, a higher morale will make you more money by being able to increase taxes. As a rule of thumb I try to stay around 65 percent morale. Be sure to either save frequently (in case you lose an election) or decrease taxes when elections near if you play with lower morale.
NOTE: Planets will grow faster by having a high morale, so early in the game many experienced players purposely lose money by decreasing taxes to 0 (or whenever the 100 percent morale hits) to grow people to tax at an incredibly faster rate.