Can you spend more than 2 gold on the House of Wonder to purchase more than one Cleric or Wizard?
No you cannot spend more than 2 Gold, yes you get 2 Adventurers.
The rules (page 5) under Assign Agent require that a building has an unoccupied action space to place your Agent. Immediately below that, the parts of a Basic Building are diagramed, showing what an Action Space looks like. On page 8, the House of Wonder is detailed, showing a single Action Space and the following text:

Action: Spend 2 Gold ( G G ), and then take 2 Adventurers from the supply and place them in your Tavern. These Adventurers can be only Clerics ( C ) or Wizards ( W ).

So, yes you get 2 Adventurers with spending 2 Gold. You cannot get 4 Adventurers, because the Action Space would be occupied by your first agent.

Assign Agent - If you have any Agents in your pool, you assign 1 of them. To assign an Agent, place it on any unoccupied action space of a Building, whether a basic Building or one that has been put into play. You cannot place an Agent on an action space that contains another Agent (yours or another player’s) or on Buildings that are not yet in play.
When you assign an Agent, follow the instructions for that action space. You take that action just once.