~is currently Gming 6~ Running more than one Rp is much like being a part of multiple Rps as just as an RPer. Depending on how in-depth the story line is, how many players in the Rp, how often updates need to be made, and so forth will determine if it is a good idea for you personally to host more than one. Each person is different, some can handle more than one and others cannot. 

I will say that you should keep notes for each of your RP's to help save time. When you are reading posts as a Gm , making a quick note about hwat someone's characters did or something you want to make sure to address in your next update makes things a lot easier. Check your Rp's daily if you can to see if any questions have been put out there that need addressing. 

Getting more than 1 off the ground at the same time is something I don't recommend but if you have 1 Rp already running well and passed that initial flood stage, then you can try a second. I stagger mine as I open them when I have spots available for hosting. Started out with one, once that was running well, started a second. If one failed, check what went wrong, make corrections, then try again. 

No matter what, don't spread yourself too thin - it is better to host 1 or 2 successful Rp's than to have a stream of failing ones. If you can manage the dedication and time for more than one, go for it but don't start a second if it is going to make the first suffer. That is a quick way to kill both Rp's with one decision and why it can be frowned upon when a Gm hosts more than 1 Rp at a time.