Russia committed circa 190,000 armed forces personnel to the invasion along with 35,00 DPR and LPR on DDay in February. In august 2021, the defence minister of Russia said the country had around 170 BTG`s >>
https://tass.com/defense/1324461 . Given probable expansion, this would have been around 190 for the entire army. Russia has been operating Battalion Tactical Groups since the debarkle in Chechnya - when large scale formations were proven to not work.
120BTG were committed to the invasion of Ukraine, taken from the Western , Central and Southern Military District (Russia has 5 commands , North, South, East, West and Central). This constitutes the bulk of Russian ground forces within 1000 miles of Ukraine. As for armour - why so many T72?`s? Russia had around 500 T80 and 350 T90 in active use, the problem being the thermal sights and a lot of the electronics are french. Once its gone, its gone - T72`s dont have the same level of problems. However that doesnt stop 1st Guards losing a regminets worth of its T80`s when it was overun. (and the odd fresh of the production line T90m, now in AFU use)