Hi I am making chunky mittens with 28 cast on I don’t want to make a stiches I do not want to make gusset thumb , how many stiches can I place on hold for a regular thumb ?

This pattern with worsted weight uses 7sts out of a cast on of 45sts. For a similar kind of thumb you could try 4sts (that would be 4sts on the lower edge and 3 on the upper with a pick up of maybe one on each side for 9sts total) and see how that works. It’s a bit of a guess, depending on your gauge and how big a thumb opening you want.