If the class rules do not prohibit multiple mainsails during a regatta and so long as you don’t try to fly two at the same time (RRS 55.1), the RRS do not restrict the number you may use during a regatta. Further, you could conceivably have a support boat bring you another mainsail between races so long as it is aboard before your preparatory signal (RRS 48.1). 

For those racing under IRC, it is prohibited by the IRC rules: "21.1.5(e) a spare mainsail may be on board but may not be used as a racing replacement, either during a race or during a regatta run on consecutive days, including any lay days. ".  21.1.5(f) may also be relevant: "exceptionally, in the case of significant damage or loss, sails may be replaced with similar sails. A Notice of Race may require that boats obtain permission from the Race Committee before replacing a sail." Both (e) and (f) may be amended by Notice of Race.

Our regional YRA has no restrictions on sail quantities either.  Boats may choose to carry a trysail which is another mainsail and the purpose is for safety so I would be surprised to see restrictions on carrying more than one mainsail.

It would be possible to introduce a requirement that the mainsail be N% smaller than the primary mainsail.  But I have never encountered a restriction like that.