Welcome in, Julie.
No hard and fast answer to your question as it will depend on the span/area of the cover, the bracing, the depth of the support runners, etc
The absolute minimum is going to be 3.5mm (1/8in) with a framed edge and bracing but that will flex and resonate unless very well made. Going for anything more than 6mm (1/4in) braced, etc, will start getting heavy. Aluminium is not very happy being constantly flexed either.
Thin metal manhole covers are the sort of design to look at (fabricated not cast) but are usually steel.