Sorry for the late response @Spyderturbo007 -- I've been out of town all week without an internet connection.
Unless you have special needs it's probably easier to you keep just one user account. Otherwise it can get confusing and annoying having to switch between accounts all the time.
Anyway, on to SMS notifications. The 25 refers to 25 total text messages -- I pay for the text messages so there has to be a hard limit. Any time you you a notification for a new game to play you will receive a text message. So for a 6 round tournament you'll get 6 text messages (maybe a few more in case there are arena updates to send).
Text messages are tied to a user account so each user has to add their phone number to their MP user account. However, as a tournament organizer you can offer text messages for tournaments you run to be debited from your account (i.e. you are paying for text messages for everyone in the tournament). This is a popular option if you have a league or lots of regulars since you'll be the only one messing with a credit card.
You can see the pricing when you go to add more messages. Currently the pricing is:
200 messages: $5
550 messages: $10
1500 messages: $25