Does this clamp come with a matching flange on both sides, or just one?

This set includes one V-band clamp and one flange. You'll need a second flange (either another V-band flange or a welded flange on your turbo/pipe) to complete the connection.

Can I reuse this clamp after disconnecting it?

Yes. The wedge bolt design is meant for repeated assembly and disassembly. Inspect the clamp band and flange seating surfaces for damage before each reconnection.

What kind of sealant or gasket should I use with this clamp?

Most turbo and exhaust applications run dry, but you can apply a high-temp RTV or graphite-based gasket sealant to the flange face if you're running boost above 15 psi or need added insurance against seepage. Test-fit dry first to confirm the clamp seats properly.