You can only have one mercenary out and helping you at any given time. There are other aspects of mercs that you can improve though:
1. Number of mercs "on call". You normally start with one merc slot. This means that if you want to switch from a tank to a healer (for example), you have to run to a merc liason, disband (or is it dismiss? The button that is now nicely colored bright red to warn you not to hit it accidentally) the one you have, and then purchase a new one. If you've purchased additional merc slots you can purchase mercs to fill them and then simply switch the merc you're using for one of the others. I think you have to wait 5 minutes between switches (it's basically suspending one and calling the other to service). But you're not usually going to need to swap mercs out that often so it's not a big deal. If you pay to play long enough you'll get a free additional merc slot. Also you can purchase them in the station store. They occasionally go on sale.
Note that additional merc slots apply to all characters on that account. You buy it once. It's well worth buying if/when you have the station cash.
2. Level of mercs. By default I think that free/silver accounts have access only to apprentice mercs (and apparently just tanks and healers). With a gold submission, you gain access automatically to the first tier of journeyman merc (J1). Not sure if that's worth the price of the sub all by itself, but in conjunction with all the other stuff you get with gold, it adds more value to the gold sub. Whether that's still worth it for you is up to you, of course. You can also improve the level of your mercs by doing the
progression quests. You don't have to do them in order. Most people skip right to the earth series and get the Journeyman 5 merc (the best level). Interestingly enough, it's also the easiest to do from a group/solo perspective. I believe every single task can be completed solo except for the final group instanced task. The other progressions actually require more group stuff along the way. So you can do everything you need and then ask if someone's running Murdunk's Last Stand (or offer to run one if others want to join in. It's worth a good chunk of exp so it's not hard to find folks to join you).
Other stuff I've probably forgotten. Check out
The EQ merc page. There are some outdated or less than clear bits on that page, but it'll at least get you started on the basics.
