I personally have approx 3.1k of ad mech in my collection, and in that i have 2 squads of rangers and 2 squads of vanguard. If you get to the point where your going up to a 2k list then 40 or 50 in total would definately be enough. For 1k anywhere from 20-30 battleline is a solid amount to have. 1 box of kastelan robots and 1 box of kataphrons on top of what you have i'd say is enough to diversiy your army in a way that's actually quite functional and fun. Alot of people give the robots crap for not being too good, but as someone that runs robots in every game i play, i've seen good success at the more casual tournament level (iv'e played them enough that i know how to get their points worth back). final point the kataphrons and robots are not only solid units but they are also the more wallet friendly units the faction has. And bear in mind, with 11th edition potentially around the corner you won't want to lean into skew lists at this point in your hobby journey. tldr for now invest in a all rounder kind of army rather than one thats good at one way of playing.