Concerning how many credit cars you can have for earning bonus miles, etc. I have 8 right now, and creditkarma.com says that my number is a little below average. I have seen reports on FT of folks having 20 or more credit cards. So, unless you have a low credit score, you can certainly go for a few. It is best to mix them up among different issuers (Citi, Chase, AMEX, etc.) and over time, but there are plenty of miles out there waiting to be accumulated.
By the way, Citi will allow you to get two personal cards at once, so there is a possibility of getting 150,000 miles on that deal. So, in my opinion that would be a VERY BIG start.