How to percentage and count in SSRS Pie Chart

1-right click on chart and select "show data label"

2-right click on label an select "series label properties"

3- general -> label data -> set this value = "#PERCENT (#VALY)"

4- it will be like this screenshot

write down an expression on series label data like -

=Sum(Fields!DATA.Value, "Chart1_SeriesGroup1") & "("  
    &  Sum(Fields!DATA.Value, "Chart1_SeriesGroup1")  * 100 /Sum(Fields!DATA.Value, "DataSet1") 
    & "%)"

Here Chart1_SeriesGroup1 is the group name if you look at series group setting

Here DataSet1 is the name of Dataset

Here is the final output

Cheers :-)