You can charge as many batteries as you like if you connect them in parallel and have them charged by a solar panel of the correct Voltage via a charge controller. If the solar panel is small, and the battery capacity is large, then the recharging may take some time. You will have to calculate the total battery capacity and compare it to the power output that your solar panel can deliver, that way you can find out how long it will take to recharge.
Say for example you have two 12 Volt, 100 Ah batteries that are totally flat. This is 1200 Wh of capacity for each battery. You have a 100 W solar panel that has an open circuit Voltage of around 21 V, which means that it’s fine to charge a 12 Volt battery bank.
You sit both empty batteries side by side, and connect the positive terminals together, as well as doing the same on the negative terminals. You now have a bank of 12 V batteries that need 2400 Wh of energy to recharge. Connect your solar panel through a charge controller, and you now have 100W being sent into the bank when the panels are in good sunlight.
In order the fill the batteries you will need that 100W to flow for 24 hours. Generally you only get around 8 good generating hours a day from a solar panel, so this means that you’ll have to wait for 3 days before your bank is fully charged back up again.