I think that with each visit (even in the same day) that uses up a plus (I could be wrong, but with the old plus features I remember being told I couldn't go to TL and BB on the same day--like hopping between waterparks, we have to stay at just one).
Here is some info I just found on Mousesavers.com:
Magic Plus Pack - This option provides the choice of admission to Disneys Blizzard Beach water park, Disneys Typhoon Lagoon water park, DisneyQuest, Downtown Disney Pleasure Island or Disneys Wide World of Sports Complex. Extra cost on top of the base-price ticket is a flat rate of $45 ($47.93 with tax). You get a set number of visits to the Magic Plus Pack attractions, depending on the length of the ticket you buy. The flat rate gives you:
1-day ticket 2-day ticket 3-day ticket 4-day ticket 5-day ticket
2 visits 2 visits 2 visits 3 visits 3 visits
6-day ticket 7-day ticket 8-day ticket 9-day ticket 10-day ticket
4 visits 5 visits 5 visits 5 visits 5 visits
As you can see, there is a big incentive to buy the longer passes, so that you'll get more "pluses" for your money. If you will be visiting the water parks and/or DisneyQuest at least twice, this add-on is definitely a good deal.
The Magic Plus Pack can be added after the initial ticket purchase. For instance, if you buy a 7-day ticket and decide on the 5th day of your vacation that you'd like to visit the water parks, you can add the Magic Plus Pack for $45 plus tax. You'll have to use the "plus" options within the original 14 days of your ticket's validity, unless you buy the No Expiration option. Such upgrades are allowed within the first 14 days (day of first use plus 13 days).
http://www.mousesavers.com/wdwticketprices.html
DJ