To find out what ornament or noble furniture you need, use this formula for mansion comfort/prestige:
Comfort/Prestige = 20 + (0.8 *x) (where x is the stat of the ornament or noble furniture)
So if you want 50 prestige, then you would say: 50=20+(0.8*x). Solving for x = 37.5. In this case, with a fraction of a point, it's safer to use a 38. However, if you plug in 37 to x, you end up with 49.6 which will round up to 50 in the mansion.
So, you need a 37 minimum prestige ornament and a 37 minimum comfort noble furniture to reach 50 comfort and 50 prestige in the mansion to be able to recruit all the available companions. However, do note that companions in the mansion are site dependent, just like all other companions. Leadership values on the items have nothing to do with availability of recruitables. The availability of recruitables depends solely on their requirement (ie prestige or comfort being met which you can check in the game guide under Leadership) and their presence in certain locations. Most people build their mansions in Whitestone or Plains of Hawkoria for ease of building and good presence.
Also, this formula works for all other buildings with bonuses. If the bonus is 10, the formula is Stat=10+(0.9*x). If the bonus is 5, the formula is Stat=5+(0.95*x). So, remember the formula and never have to ask "What do I need to reach X comfort?" again.