w3 can handle around dozens of thousands of units at the same time but the game will be badshit lagged and require shitload of ram to run, also if you have too many units ordering to move etc the units will start moving glitchy when you have more then 150 units, at least in 64x64 square area Ive experienced that myself
Why I know that?
I have kept creating location and unit handle for every 0.00(for every time the system is able to, so always) for around 3 minutes until wc3 ran out of space in handle table and at that time it was taking 1,1 giga from my ram so yes, wc3 can handle more untis than you will ever use.
If you want some practical example, check theazothen's(yt channel, he propagated it here on hive) replays of the azeroth wars: legacy reborn, you have 12 players at same time playing each having around up to 200 units on map and the game wont even lag on new mashine, plenty to use