It depends on how strong you want the raspberry flavor to be. For my blueberry wheat I use 1lb of frozen blueberries, leave them in for about 5 days then rack to a tertiary for a few more days to let the remaining blueberry debris settle out. This gives a nice light blueberry taste, not overpowering at all. If you want a stronger berry taste use more berries or just leave them in longer.
To prep my berries I basically pasteurize them on the stovetop. Just put them in a sanitized sauce pan with a little water and hold them around 170-180* for about 15 minutes, let them cool then rack onto them in the secondary