You don't need to double that number because this is going to be combine arms (well, at least that is what we all thought) and combine arms attack usually apply pressure on weak point, Russia had around 200,000 soldiers along the entire frontline (most likely less than that now, but let's just say that it does), all you do is to find a weak point where the enemy concentration is in their favor and attack there, a gap with 10,000 or 15,000 defender within the entire 1200 km frontline and push thru there, you don't need to pit those mechanised brigade against the entire Russian frontline.
The West don't fight the way Russian do where they take town center, Combine arms operation aims at taking the road leading into the town center and surround/bypass the town. You punch thru the line with your armour and then drop your mechanised infantry to encircle the town, and then you move on.
What people don't realise is, by defending Bakhmut, Ukrainian is saving their own mechanised force, I mean Bakhmut defence is statics and in the business we call them "Low Cost Defence" because you don't need to use IFV, Tank, Artillery or Aircraft much, you simply put troop in a city to grind down the enemy. Yes, the lost would be appalling for Ukraine, but you save the equipment. Which mean beside what the West gave them, now they have another 3 or 4 of their own Mechanised Brigade that can be put into the fight with the newly formed Western Arms brigade. So instead of mostly Western Tank (Leo 1, Leo 2, Challenger 2 and Abrams), you are going to see some T-72 and T-80 in the mix for the upcoming action. With BMP and Bradley/CV90 bringing in the mechanise infantry.
