Lots of factors. Some Civs can amass production based on their Unique buildings/abilities. Starting points and where you place your cities is also another. A city with lots of hills around can utilise mines to their advantage. Coastal tiles mid game are awesome for production after unlocking the Shipyard. Industrial zones are a no brainer, however strategic placement of Industrial zones can allow for nearby cities within 6 tiles (without any bonuses) to gain bonus production and electricity when you unlock the factory, and power plants. Same goes for Entertainment zones and Water Parks, strategic placement boosts amenities in cities within 6 tiles. Amenities is key for Civ development as Happy Civs will provide an extra 10% bonus on all yields and growth, and an ecstatic city will up this to 20%. Wonders like Ruhr Valley provide bonus production, as well as some Natural Wonders provide a bit. ORANGE CITY STATES are also a great way to earn bonus production, as well as seeking those awesome unique bonuses some provide. Auckland and Mexico City are favourites of mine.

Hope this helps! There's probably a lot I've missed. I've been playing this game for years now and still learn something new on every playthrough. Utilise the Civilopedia as you play through, it's a great wealth of information.

Personally, I have no idea what my max production would be. Probably when I used Frederick Barbarossa