A 6th-level ranger with Practiced Spellcaster has caster level 6th, that much is true. But he still has the spells per day of a 6th-level ranger: one 1st-level spell. Practiced Spellcaster has no effect on spells per day (or spells known, for the classes that have specific spells known at each level; the ranger is not one of these classes).
[Practiced Spellcaster] does not affect your spells per day or spells known. It only increases your caster level, which would help you penetrate SR and increase the duration and other effects of your spells.
When the ranger becomes a 7th-level ranger, his caster level would also increase to 7th—still 3 from ranger levels, now +4 from Practiced Spellcaster. He still only has one 1st-level spell. At ranger 8th, caster level 8th, 4 and 4, and now he has “0” 2nd-level spells, i.e. he is eligible to get bonus 2nd-level spells per day from a high Wisdom score. And at ranger 9th, his caster level remains 8th—since Practiced Spellcaster can give at most +4 to a class, he gets 4 from his ranger levels, +4 from Practiced Spellcaster, for 8th total.
To get the spells of a 12th-level ranger, you will have to actually be a 12th-level ranger, or at least have 12 effective levels of ranger spellcasting from ranger levels and prestige class levels that progress ranger spellcasting. There is no way around this in the entire game.
You can improve ranger spellcasting, though
While a 6th-level ranger will always have the spells per day of a 6th-level ranger, and nothing will ever make him have the spells per day of a higher-level ranger, you can change the spellcasting of the ranger class. In Dragon magazine volume 336, there is the “mystic” variant to the ranger. This version gets favored enemy and the combat style bonus feats at later levels than a regular ranger, but gets more spells, at lower levels, than a regular ranger does. This is often recommended, since spellcasting is far more important than favored enemy or bonus feats, and besides, you still get those, just a little later is all.