Nobody actually addressed the fact that except in very rare cases, balance transfers to not earn any miles. So you could spend whatever you like, but will probably only get miles 1 time - so you should spend on the ones with double miles, not try a balance transfer. And balance transfers often (not always) have large transaction costs associated with them.

And prncess674 - the Fair Isaacs folks who create the FICO score say that actually having more lines of credit open is a good thing (if you aren't using them), because a prime factor in calculating the score is the percentage of credit that you have that you are using. I don't know how other creditors look at multiple accounts, but I know the Fair Isaacs president said in an interview that there is a misconception about open accounts.