The ‘STATIC’ variable must contain the exacct text that you want to search for. That same text will be removed from the output of grep.
STATIC='subreddit_subscribers": '
curl -s -H 'User-Agent: custom' https://www.reddit.com/r/askreddit.json | \
grep -o -E "${STATIC}[0-9]+" | sed "s/${STATIC}//" | head -n 1