Roth TSP is NOT the same as Roth IRA. The thing that's the same about both of them is the tax treatment -they will grow tax free forever.
In general (exceptions and caveats apply):
IRAs have limits of $5000 per person per year
TSP has a limit of $16500 per person per year
Right now you can choose to have an IRA as "traditional" which is taxed when you take the money out or "Roth", which is tax free on withdrawl.
Right now, you can only make contributions to TSP as "traditional". Starting in 2012, you can choose to make contributions to TSP as "Roth" if you want to.
Hopefully this explains it. If unclear, the answers to your specific questions:
It doesn't matter how many IRA accounts you have, you are limited to $5000 per year per person.
IRA and TSP are not mutually exclusive. You can contribute to IRA and TSP whether it has Roth tax treatment or not.
Finally a comment: You shouldn't be putting money into TSP that you want to take out before you leave government service. After you leave the service, you can take the money and roll it over to an IRA or if you wish, you can buy lottery tickets with it (minus taxes and early withdrawl fees if applicable).