Per FAR 43.301, the SF 30 is the correct form for your contract action.  You are describing a unilateral admininistrative change as described in FAR 43.103.  

I saw nothing in the incructions for filling out that form or the FAR that prohibits using one document to modify multiple contracts, but in most cases it would be impractical.  Take for the example the modifiction numberin block 2 of the form, you would likely have a list for that block.  P000xxx for contract C-xyzzzb, P0000yyy for contract C-jeben etc. In fact most of the blocks would be that way (see attachment x), so this proposed "megamod" would come with pages of attachments that would likely just add more confusion than writing individual mods.  And I would never consider using this for more than one contractor.  And when you think about it a bit more the contracts with that contractor could be managed out of different offices like San Ramon, Cherry Hill and San Antonio.  So you now have a distribution problem potentially on the contractor side.