No other (maybe more serious) input to this thread?
A more serious consideration is what difficulty are you playing at? That'll change what you can expect from the AI in terms of resistance via its units and what not.
Just before Education, an ideal group for me (Emperor diff. normally) would be 4-5 CBs, 3 Catapults/Trebuchets, 2 Pikes, and 2 Horsemen or Scouts. The CBs will help keep your other units from getting attacked by too much all at once, while the 3 catapults will bring the city down quickly, and the 2 Horsemen or Scouts can be on hand to rush into the city and finally take it after other units bring it to zero. Now, you may ask, why the 2 pikes and why 2 of either horsemen or scouts. For one, sometimes the AI may take the city right back (especially if you stay on one side of the city and can't see the other, back side of it). That way, if they take the city the turn immediately after you took it, you can still take it back right away with another volley from the catapults and your last horseman or scout.
The 2 pikes I bring along to throw in front of the whole group should it appear the enemy is going to have units to counter attack you as you get in position.
But there's lots of other factors. A city which is on the other side of a river may have a low hp or defense value, but the river adds complications: trying to encircle the city will be harder, and any melee units attacking from the opposite side of the river will take a lot more damage. Also, a city nestled in between mountains, or with a bunch of hills around it, will be likewise very hard to encircle.
So, TL;DR --> The attack force needed to take a city is usually pretty situational. Assuming the city you want to take is in entirely open ground with no rivers, the force will be smaller by far than if the city is on the opposite side of a river from you. Or is between two impassable mountain tiles and has forest or hills for the only tiles you can get next to the city from. It varies a lot. Add to this if the enemy has the Great Wall, or if you have the Statue of Zeus, or if either side has a Great General, and things get even more complicated in terms of what you absolutely need at a minimum.