It could happen 2 twice easily as there is a draw advisor at levels 4 and 7 and then 13
There is also a development matrix token that gives another advisor so a player can draw all 4 from their deck.
If a player lose a level (technology or military) he also lose the ability associated to the level but not the bonus gained. Additionally, once the lost level is gained again the ability and the bonus are gained again. For instance a player has technology level 16 and lose one level; when the level 16 is gained again 6 VP are scored (for the second time).
I have just seen this in the rules as well.
Rules, Page 9. Advisor Cards wrote:
When an Advisor card is used, it is placed face-up on the bottom of the Advisor deck. If the top card is face up, reshuffle the deck and place it face down a new Advisor deck Emphasis mine.
So it would appear that you can get more than 5 Advisor's in a game.