In a single asm_diskstring parameter we can have 255 characters and this excludes the asterisk (*) , comma (,) and single-quote ('). This is limited by v$asm_disk.path column's datatype length.
In an ASM instance we can have multiple asm_diskstring parameters. But the values from them are combined. And the combined string value is controlled by v$parameter.value column's datatype length that is 4000. Though it will not error, the value of the Nth asm_diskstring parameter, that is falling after 4000 characters will be ignored.
From Oracle
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