As there are others giving really good advice on the water heater, might I suggest installing ultra low flow {as in 1.5 gpm} shower heads in the house. Then, you can get more minutes of shower usage out of a tank of water. On those infrequent occasions where there might be 4 in a row, you're better covered.
Also, let me throw this out there. I live every day with a standard 50 gallon natural gas HWH. My wife and I could run the dishwasher, both of us take showers and our daughter take a shower and we never ran out of hot water. {The daughter is now married and gone.} And that was before I changed to a 1.5 gpm shower head in the master bath. Gas seems to recover very quickly, but that's a very unscientific personal observation.
