Having copied a spell into his spellbook is - for a wizard - the same as knowing the spell. That's my understanding at least.
When exactly does a spell counts as 'known' for a wizard?
Wizards don't really have spells known, they're prepared spellcasters.
In cases where the rules mention spells known for a prepared spellcaster, I think it's reasonable to assume it means a spell that is recorded in his spellbook (or formula book or familiar, etc.).
However, under Wizard Spells and Borrowed Spellbooks it says "A wizard can use a borrowed spellbook to prepare a spell he already knows and has recorded in his own spellbook..." which could be read to imply that knowing the spell is not the same as having it recorded in your spellbook.
I assume this is a question of whether or not a Wizard can use Secret Page to change the spell in his spell book to be a different spell he's not yet learned... and then study that new spell to learn it? No, that's a bit broken.
With Wizards it is considered known if he's passed the "learn the spell" check to learn from a written source (scroll or another's spell book) or has picked that spell as one of his free on-level-up spells.
So in this case, the wizard is not limited to any number of spells, but Secret Page can only be used to write out spells he has already learned, not some random spell he didn't previously know. If he can prepare it (assuming access to his own spell book), then he can use it with Secret Page. He can't use this spell to magically acquire any spell in the game.
Wizard Sevta knows fireball. He knows that he has it in his book back at his house.
He knows!
But he forgot to prepare it, he forgot the last few motions and flicks, and is found cursing by the zombie horde that now surrounds him...
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A wizard begins play with a spellbook containing all 0-level wizard spells (except those from his prohibited schools, if any; see Arcane Schools) plus three 1st-level spells of his choice. The wizard also selects a number of additional 1st-level spells equal to his Intelligence modifier to add to the spellbook. At each new wizard level, he gains two new spells of any spell level or levels that he can cast (based on his new wizard level) for his spellbook. At any time, a wizard can also add spells found in other wizards' spellbooks to his own.
So the spells that a wizard would know would be the ones he learned by leveling up, as well as those he's copied from other wizard's spellbooks into one of his own.
As a GM, I would not allow "I do know it, I've just never gotten around to copying into a spellbook" as a reason for allowing new spells to be written via Secret Page.
"It's in my other spellbook, which I left back in the tower", however, would be. Provided that he indeed leave that other spellbook in the tower.