Yes, easily at that time. In its hey day, before the Normid and the narrowing of the terracing at the back, it held over 20,000. Im sure when Newcastle came in the FA Cup in the seventies, they brought 17,000, and Bolton fans were also present in the Embankment then, obviously that was before the decrease in capacity.
PS just googled it, the capacity was 28,000 when at its biggest.