2MB is the limit for Chip RAM. Every A500+ has 1MB onboard and can take an additional 1MB via the trapdoor. Like zipper mentioned, you need a 1MB trapdoor expansion suitable for the A500+. There are lots of these on the market, vintage and new, like the one I linked (Individual Computers, maker of the ACA500plus, also has one). Just make sure it's an A500+/1MB expansion and not an A500/512K expansion. If it has a clock/battery on it it's for the plain A500 - the A500+ plus has a clock on the motherboard and so its trapdoor expansions don't include one.