The SE/30 shipped with 1MB, 2MB or 4MB RAM as factory options. At launch, it was quoted in sales literature as being compatible with up to 8MB RAM. The intention was always that more than 8MB would be usable when SIMMs in sizes higher than 1MB were made available.
As SIMMs in higher sizes were made available later, the SE/30 could eventually support up to 128MB RAM, via 8 x 16MB SIMMs.
RAM was typically installed by an authorised Apple dealer, as the SE/30 was not user serviceable in an official sense, even though the RAM is socketed so it's a fairly trivial upgrade for someone familiar with opening a compact Mac.
The SE/30 logic board has two banks of memory slots, 8 in total. The SIMMs in each bank need to be of matching size and speed.
Which RAM is compatible and where do I get it?
The memory supported is 30 pin SIMM @ 120ns.
Today, the SIMMs are still available from OWC:
Memory upgrade guide