A question here reminded me that I've been playing on Normal speed instead of Faster during the Practice games I've been playing. I'd like to instead practice at the more oft used speed of Faster in preparation for actual games. So, can one get "more" practice games?
You can't get anymore 1v1 practice league games, but you can play custom games that don't count towards your placements.
If you have a friend who has practice games you can sign up together for 2v2 practice league.
Once you have exited the practice league there is no way to go back.
Your best bet for practicing would be to play custom games vs a computer on faster speed or to go through the campaign (you can change he speed to faster through the menu -> gameplay -> there should be a slider for gamespeed if you push it to the right it will turn it to faster).
The practice league isn't that helpful for much other than learning what buildings make which units which can be taught just as well in custom games vs easy AI. You can make games with YABOT (yet another build order tester) maps to learn common build orders for your race to prepare you for the ladder as well. These can be found by creating a custom game and searching for YABOT when choosing a map.
Once you're comfortable with how everything works hit up the ladder and get the rest of your practice there! You may win or lose but either way you get to learn how others play and where you can make improvements.