To start Hotseat game with more than 12 players you have to change default number of civ per world/map size. You can change it in file: .../gameplay/xml/gameinfo/ CIV5Worlds.xml. In this way you can start hotseat game with max 22 players.
-<Row> <Type>WORLDSIZE_HUGE</Type> <Description>TXT_KEY_WORLD_HUGE</Description> <Help>TXT_KEY_WORLD_HUGE_HELP</Help> <DefaultPlayers>22</DefaultPlayers> <DefaultMinorCivs>11</DefaultMinorCivs> <GridWidth>128</GridWidth> <GridHeight>80</GridHeight> <FogTilesPerBarbarianCamp>35</FogTilesPerBarbarianCamp> <NumNaturalWonders>7</NumNaturalWonders> <UnitNameModifier>0</UnitNameModifier> <TargetNumCities>6</TargetNumCities> <NumFreeBuildingResources>7</NumFreeBuildingResources> <BuildingClassPrereqModifier>100</BuildingClassPrereqModifier> <MaxConscriptModifier>75</MaxConscriptModifier> <TerrainGrainChange>1</TerrainGrainChange> <FeatureGrainChange>1</FeatureGrainChange> <ResearchPercent>130</ResearchPercent> <NumCitiesUnhappinessPercent>60</NumCitiesUnhappinessPercent> <NumCitiesPolicyCostMod>7.5</NumCitiesPolicyCostMod> <AdvancedStartPointsMod>120</AdvancedStartPointsMod> <IconAtlas>WORLDSIZE_ATLAS</IconAtlas> <PortraitIndex>5</PortraitIndex> </Row>
-<Row> <Type>WORLDSIZE_HUGE</Type> <Description>TXT_KEY_WORLD_HUGE</Description> <Help>TXT_KEY_WORLD_HUGE_HELP</Help> <DefaultPlayers>22</DefaultPlayers> <DefaultMinorCivs>11</DefaultMinorCivs> <GridWidth>128</GridWidth> <GridHeight>80</GridHeight> <FogTilesPerBarbarianCamp>35</FogTilesPerBarbarianCamp> <NumNaturalWonders>7</NumNaturalWonders> <UnitNameModifier>0</UnitNameModifier> <TargetNumCities>6</TargetNumCities> <NumFreeBuildingResources>7</NumFreeBuildingResources> <BuildingClassPrereqModifier>100</BuildingClassPrereqModifier> <MaxConscriptModifier>75</MaxConscriptModifier> <TerrainGrainChange>1</TerrainGrainChange> <FeatureGrainChange>1</FeatureGrainChange> <ResearchPercent>130</ResearchPercent> <NumCitiesUnhappinessPercent>60</NumCitiesUnhappinessPercent> <NumCitiesPolicyCostMod>7.5</NumCitiesPolicyCostMod> <AdvancedStartPointsMod>120</AdvancedStartPointsMod> <IconAtlas>WORLDSIZE_ATLAS</IconAtlas> <PortraitIndex>5</PortraitIndex> </Row>