Ken, you can use two simple sql commands to get this done as follows:

select custno, cnt(*) as prchasecnt ;
from purchases ;
group by custno into cursor PurCntCurs

select ;
sum(iif(between(prchasecnt,1,5), 1, 0)) as pur_1_5,;
sum(iif(between(prchasecnt,6,10), 1, 0)) as pur_6_10,;
sum(iif(between(prchasecnt,11,15), 1, 0)) as pur_11_15,;
sum(iif(between(prchasecnt,16,20), 1, 0)) as pur_16_20,;
sum(iif(prchasecnt>21, 1, 0)) as pur_20_over ;
from PurCntCurs