The option to convert SBD to STEEM has recently been removed from steemit.com (the UI) because of the high price of the SBD. Converting to SBD is not the same as selling to SBD. Selling is still posible in the internal market. The reason converting has been removed is that selling gives you currently a lot more STEEM.
SBD has no fixed supply.
You can find the current STEEM and SBD supply on my block explorer website: https://steemblockexplorer.com. Search for 'current supply' and 'current sbd supply'.