What are the dimensions of the tank? You might be ok but at or near capacity, depending on the dimensions. Platys are high bioload fish and need a good sized tank for a large group. Most people say neons need at least a 20 gal
since they are active fish but they might be ok in your tank. At most I'd add 6 neons until you get a larger tank, but this would depend on the size of the tank (e.g. if it's the dimensions that Tomatt put in, I wouldn't add anything else. If it's larger, say 24" across, you could do the neons). 65 liters is actually about 17 US gallons so you might be ok with the neons unless it's an oddly shaped tank like a hex or column.