I just bought a model 29-5 classic I need to buy a rear sight does anybody know how many screws it takes to hold the rear sight on the frame ?

I'm not sure, but you can probably get a manual from S&W that will have a schematic for your gun and that will show you what you need to know. You also may be able to get a schematic from here if you can't download a manual from S&W.

Firearm Parts and Accessories | Numrich Gun Parts Corp. | e-GunParts.com (http://www.e-gunparts.com/dept.asp)

You can buy all the parts you need from Smith and Wesson.