Normally, the staffing should match the call volume change. Especially at the peak time. That the reason why a lot of call centers arrange people online by every half hour.
At your case，（required to have 50 people on the phones every hour regardless of the call volume），the key issue here is working shift, you can have 50 agent work from 8am to 12pm,(4 hours per shift),12 pm to 4pm, 4pm to 7pm Which means you need 150 person for three shift(4 hours per shift, 3 hours for last one).
Given the sick leave, training, vacation, you need 50 x 3 x 1.5= 225 agent in pools.
Meantime, you also could have 2 shifts, every shift is 7 hours, then you need 100 people every day.
Given the sick leave, training, vacation, I will suggest you need to have 50 x 2 x 1.5 = 150 people to in pool.
This is only the analysis for your situation, there are also some operational for this, for example, you need to have a monthly staff forecast plan for this, including all the situation, such as sick leave, vacation, training and other out-of-office situatinon.
You may also check my web-site frequently for detail information at
http://www.callcentersavvy.com , Where I will post a lot of call center operational knowledge and tips there.
Hope this helpful.