gen x3=x1
collapse (count) x3 (mean) x2, by(sample x1)
gsort sample -x3 x1
by sample: keep if _n<=5

bys sample x1: egen meanx2=mean(x2)
bys sample: egen countx1=count(x1)
duplicates drop sample x1 countx1, force
bys sample: egen rankx1=rank(countx1), unique
keep if rankx1<=5
sort sample rankx1
keep sample x1 meanx2

another way:

tempvar c
egen double `c'=count(1), by(sample x_1)
egen double m=mean(x_2), by(sample x_1)
gsort sample -`c' x_1
keep if x_1!=x_1[_n-1]
bysort sample : keep if _n<=5