I think the first question is whether or not these rooms do need to be separate subnets or not. If they are just a set of rooms in one building for example I'd probably say no, unless they werefor seven separate companies or departments that need segregating from one another.
Assuming they do need to be separate subnets then you'd go through some logical exercise to find the maximum number of PCs that might be required in any one subnet. That number is what the hosts part of your subnet mask needs to accomodate. Once you know how many hosts bits you need to support the number of host PCs you want then working out the rest of the mask is easy
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