I currently have a 23 year old water heater in an 8 unit building. The inlet and outlet areas on the water heater are corroding, and when last checked, appear to have water pooled around them. I would rather have it replaced before a full blown leak occurs. 

Looking online for a new water, I'm not sure of what to go with. It currently runs on natural gas. Should I go with a 98 gallon heater or will a 75 gallon suffice? 

Wouldn't you replace it with the same size tank that it currently has? That is if the current size tank is supply an adequate amount of hot water for the units.

@Kyle Lecy

Even if the apartments are all 1/1, you might want to go with the 98 gallon or for about the same price you could buy a gas tankless 9.5-11 GPM. The average shower uses about 2 GPM or 17 gallons per shower. If 5 people shower in a 60-75 minute period the water will go cold (with a tank); not counting other hot water activities.

What size is there now? Id replace it like for like if there was plenty of water before. The only way I'd drop the size of that tank was if you have a boiler and can do an indirect tank. 

Currently 100 gallon. I'll keep it the same. Thanks for replies!