About what percent of mutations are not adaptive?
As suggested by the McGrew comment, this is dramatically dependent on the shape of the fitness landscape. It will vary quite a bit from situation to situation.
More precisely, you are interested in the distribution of fitness effects (DFE) of mutations, and the upper tail specifically.
Wikipedia has a section on this. In there, they note one study of a virus that suggests that ~4% of mutations are beneficial. That may be an overestimate, and many of those are probably only very slightly beneficial. Probably the number is lower in non-viral organisms.
For a review on the DFE of new mutations, see here. They write in their section &quot;Advantageous mutations&quot;:

As expected, relatively few of the mutations that are not
effectively neutral are advantageous.In three mutagenesis
experiments, the proportion of advantageous mutations was 4% in the RNA virus vesicular stomatitis virus (VSV)15 (FIG. 1), 0% in Escherichia coli(14), 0–15% in the bacteriophage φX174 (REF. 40), 0% in φ6 (REF. 13) and 6% in Saccharomyces cerevisiae (16).