What's the conventional wisdom on the number of AUs that can be carried at any one time? I have a Citi card listed with two companies and could end up with 4 adds total, though I've never gone above 2.
Old Company has recommendations for number of slots that vary by issuer. I always follow their recommendations, although you can perhaps choose to have more slots if you're willing to take the risk and the TL company you're using is willing to go along with it.
Occasionally the Old Company (and maybe New Company as well, I can't recall) will have situations where they want or need you to temporarily exceed your number of slots. This is usually temporary. I decide these on a case-by-case basis, but I decline more often than I accept. When I decline, Old Company has not had an issue with it.
If you're listing with multiple companies, then you're taking a risk, one that I personally find excessive but reasonable people can disagree. You might average more sales in the short term, but you're increasing the risk of either (a) getting the card shut down, or (b) choosing to decline an order to manage your risk. If you do either of these too much and the TL company figures out what you're doing, I think they're likely to dis-enroll you.
The established wisdom with all games of this type is to milk it at a reasonable level. Going too fast typically results in higher results for a shorter period of time, but the cumulative total is typically less. Going too fast also tends to accelerate the game getting restricted or shut down entirely. This is why I follow all of the recommendations from the TL companies - to keep the game going for everyone, and to maximize my total lifetime income from the game.