Some would suggest that you could twin up the existing 20's but I would put in a panel with more space. If you panel is one of those exposed in the basement units, it is actually not difficult to do......for me anyway
The existing panel is already crammed with wires and adding a few more won't help.
Cost for a new panel and breakers will be under $200
If you are a good mechanic it is just a matter of taking out the old and putting in the new. There are a handful of rules to apply. Post pics and someone will (maybe) walk you thru it.
If you are not so good you might want to call someone.
*permits, inspections and power company disconnect may apply