How many stoners is 290 pounds in weight?

First of all it is 'Stones' , NOT 'stoners'.

The Imperial Tabler of Weights is

16 ozs = 1 lb

14 lbs = 1 stone ( Use this line from the table.)

2 stones = 1 quarter

4 quarters= 1cwt

20 cwt = 1 ton

290 lbs/ 14 = 20. 7142 stones or 20 stones & 10 lbs.