Just for grins...
Variable. You have to have a few elements, assuming this is a full up combat operation.
1. Security element. Enough people to secure a perimeter around the objective so nobody from the outside can get into the fight, i.e. your target's buddies from three blocks down coming to help.
2. Cordon element. Can be the same as security, make sure nobody on the objective can escape.
3. Overwatch/support element. Provide fire support for the entry team, can be snipers covering or a machine gun team sweeping the objective ahead of the entry team.
4. Entry team. Sub elements can be breach team, entry/foothold team, and a third team to leapfrog. Command and control element is basically a traffic cop, directing people. Once in the objective, all team members have to be trained on all positions, as teams will become split.
5. Quick re-action force, or QRF. Team that can re-inforce any of the above teams in a critical situation.
ETA:
Few high speed tips.
1. Assualt from top down, if possible. Gain entry from the roof via helicopter, ladder, etc. That way the enemy never gets trapped, tries to run and is gunned down by elements outside.
2. Never go through a door or window if you can make your own hole.
3. Violence of action. The longer the assault takes, the more chances you will take some casualties.