Only Sheila would have known that. She'd have loaded the bullets one at a time anyway and would have lost count.
How could she lose count - it's 5 not 305? anyway .....
How does Sheila going to the cupboard for ammo pan out? Jeremy said he loaded between 8 and 10 bullets, so that would be all she had to start with. To reload (given that she had no idea that there was a box open on the counter top), she would have to go to the cupboard in the study to get the '5' bullets. If we take it that there were 10 in magazine to start with, how did she manage to kill the twins and disable June and Nevill and get to the cupboard in the study, find and reload the 5 shots? There were 9 shots in the twins room alone, 13 in the main bedroom, 1 on the stairs and 3 in the kitchen. No shots could have been fired prior to Nevill's supposed call so ....... 10 bullets and time to look for more in the cupboard/study?