how to create cavity using multiple surfaces?
i have step file which has around 200 surface
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As CadCloud pointed out, you have not provided enough information to visualize, let alone answer the question.
I'll take a guess though.
Use the Knit command to knit the ~200 surfaces into a single body. Further repairs will likely be needed as well before you can use the Thicken command to convert the surface body into a solid body.
To form a cavity via a boolean operation, there are a few tools.
Combine (with the subtract option) work well.
I often use Indent as it allows for some different options.
Intersect can sometimes be used, but I find it confusing.
Cut with Surface is an option in some cases.
Also, you might check what your import settings are for Step files. I think there is an option which tries to merge/thicken surfaces on import. Maybe that would solve the ~200 imported surfaces?