Stacey wrote:

Hi, I need to know how many stops of light I will lose using a B+W 091
dark red filter?

3 stops. I've also found that some B&W films might need even more exposure
to get printable negs using this type of filter so I'd bracket another
stop
over and look at the results.

Depends on type of light too.
In artificial light, there is proportionally more red, hence a larger part
will be filtered out, which in turn means more compensation needed.
In daylight, there is ... You get the picture. ;-)