How many screws should the screw-maker make?
They probably get asked every day for some screws. They are either capable to extrapolate this into a reasonable monthly goal or they ask for help from someone who is good at that. If they make a mistake and make not enough, than people who need screws will probably have to wait a little. And if they make to many, they can spend their time doing something else. Like where actually is the problem here? We dont need perfection in an anarchist society.
They could even create a guild or syndicate or something with other screwmakers to help each other out in materials, when one is unable to work, when a communities needs emergency screws etc.
And if screw making to meet everyones need is so shitty that nobody would want to do it, then everybody who is able to should do part of it and as a community think of a process that makes this possible and maybe even good by whatever metric they care about.