Do I need anything else to seal the connection, or does the V-band clamp alone create the seal?

The V-band clamp itself creates the seal through clamping pressure — no gasket or sealant is required. The mating flanges must be clean and flat for the best seal. If you're running high boost or want extra insurance, some builders add a thin layer of high-temp sealant, but it's not necessary.

Can I reuse this clamp if I disconnect and reconnect the pipes multiple times?

Yes. That's the whole point of V-band design. You can pull it apart and reassemble it as many times as you need without damaging the clamp or losing clamp load, as long as the flanges stay clean and undamaged.

What torque spec should I use when I first install this?

V-band clamps are hand-tight by design — you tighten the bolt by hand until snug, then give it a quarter turn more. Refer to your turbo kit or exhaust system documentation if it specifies a different procedure for your particular application.