A last minute addition to the HF was added to increase custom json from 1 to 5 per block, this will greatly help apps that use custom json. It was limited to 1 previously when there were no RC's to limit spam, it is safe to increase the limit now that RC's exist to regulate it, also each custom json can only be a max of 2048 bytes in size as regulated by Jussi and 8192 bytes as a blockchain hard limit.
Steemit Inc issued this post about it.
https://steemit.com/hf21/@steemitblog/hf21-recommendation-raising-custom-json-limit
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