255 characters with the base UI. 1023 if you are using Macaroon.

How many spells/abilities are you setting up and for what classes? I generally find I only need one DPS macro (or two if I decide to change them up based on situation). Healing spells are a bit different since those tend to be bound to a specific character and not what you're trying to kill.

Alternatively, if you don't want to include a [target=focustarget] in all your offensive macros, expecially ones that only have a single ability, you could put /assist focus at the begining of the macros you use a great deal, so that your alts stay targeted on whatever they were going to hit anyway.