How many baby reds in a 180??

Hi... I ordered a 180 all glass brand fish tank(72x24x25) today and will be getting it in about a month... I know 8-9 full size reds would be a good fit for life. I've had reds before, I just traded off some bigger ones to clear a tank that I am selling to help pay for the new one. I want to get small ones to start it with this time.... I have a pet store that will sell them to me quarter size for $4.00 a piece. I know they are very aggressive towards one another when small. How many do you guys suggest putting in there to start?? I was thinking like 15-20 and 6-7 monthes from now the weak ones should be weeded out. Think I will have 8-9 good ones out of that many or should I get more or less than 20 to start? Thanks!! Jake