Dependent on your tolerance, one or two drinks is good, but don't let it snowball. The mood of the celebration is quickly dampened if the maid of honour flubs her speech, goes on a rambling tangent, makes inappropriate comments, or throws up from a combination of anxiety and drink. A maid of honour speech doesn't have to be a comedy act if humour isn't your strong suit. Be as emotional, sweet, or nostalgic as you like, but don't get intoxicated with it. It would help if you held off on drinking until after the maid of honour's toast.