I have a metal shop that I built a room inside with lumber and sheet rock. I am going to run 3/4 emt to a large metal juntion box that I have six romex 12/2w/ground wires from the wooden structure inside. Can I run one ground from the main panel to the juntion box and attach it to all the romex ground wires via ground bus that is also attached to the metal box? Should I run a ground wire for every circuit from the Panel to the junction box and if I do do I need to go larger on the EMT? Only five feet away from panel to Junction box.

Do not need separate grounds, one in the conduit and spliced to the six from the cables is fine. Just needs to be sized according to the largest circuit. In this case, since they are all #12, I will assume all breakers are 20 amp, so minimum of one #12 is sufficient.

This is a nit-picking technical detail. The issue will be derating. Bundling current carrying conductors over two feet requires derating. 6- 2 wire cables is 12 current carrying conductors. Need to derate the #12 by 50 %.

The conductors of Romex needs to use the 60 Deg C column of Table 310.16, which is 25 amps. I assume you will strip the sheathing off the cable long enough to pass through the JB through the conduit and not have splices in the JB for the blacks and whites. This means you could only protect the wire with a 15 amp circuit breaker. 1/2 of 25 is 12.5, round up to nearest standard breaker size, 15.

Technically, you should be using #10 THHN wire in the conduit, and splice to the Romex cable in the JB. Then you can use 20 amp circuit breakers.

Or, run another conduit, and have three cables in each, strip the cable long enough, pull the wire through the conduit, use 20 amp breakers, and eliminate the splices in the JB.

Thank you for the response and info. You brought up more questions from me. I need to give more info. The room will be used as a game room Computer, TV, Lights so will 15amp breakers work? 15amp is what I was planning on. I have to splice in the box because the romex is cut there. I had the 12-2 w/grd wire that's why it was used. I have not run the conduit from the panel to the juntion box yet. What size conduit and THHN wire should be used. I have 3/4 conduit and #12THHN wire but I want to make sure it is correct before I run it. Thanks again.

If you use 15 amp breakers, then fine, the #12 will be derated, and you can use the 13 #12 in the one 3/4 inch EMT.