You can only have 2 wonders built at the same time in 1 town, if you wish to have more wonders you will have to build them in other towns. You cannot build two wonders across the civ at the same time as well, you need to have a wonder finished before you can build the next one.
Having 2 wonders in 1 town at the same time will grant you an additional buff.
Buffs are listed below:
|War Defensive||War Offensive||Economic||Production||Scientific||Cultural|
|War Defensive||All your Towns' Control Blocks gain 50% bonus Hit Points from Era scaling.||+4 Health permanently to all Residents.||Gives you +1% income rate per Tower Level unlocked in the host town.||If you're involved in a war and you're not the aggressor, your Civilization has +35% Hammers toward defensive structures.||The host town can build two more additional towers by your choice.||Every 10 seconds, heal your Residents inside your own borders by [0.125 * Host Town's Culture Level] Hit Point.|
|War Offensive||During war, damage against Town's structures have 50% chance to be increased by 1.||When you pillage a town, your civilization gain coins equal to its upkeep.||+15% Hammers toward Units and Catalysts in all Towns.||Once you pillage a town, you gain 2% Beaker Rate per town (up to 16%), and if you pillage a Civilization, it counts as every town until pillage is over.||Once you pillage a town, you gain 2% Culture Rate per town (up to 16%), and if you pillage a Civilization, it counts as every town until pillage is over.|
|Economic||Increases Income Rate inside the Civilization by 15%||Hammers that are not used in this Town are converted to Coins.||Static scientific structures give +3% income rate toward the town.||You convert 3% of your Coins to Culture in the host town.|
|Production||+15% hammer rate across the civilization.||Reduce Wonder's Research Cost by 30%. (Coins, Beakers and Culture)||During Golden age, increase Hammer rate and Culture rate by 25%.|
|Scientific||You can upgrade 3 structures to their Wonder Upgrade level besides your current Wonder Upgrade level in the Host town.||The Host Town gain +25 culture and beaker per culture zone and is affected by it's rate|
|Cultural||The host Town gain 1 Culture Zone for every 3 Culture levels.|