One S7-300 row can have max 8 modules.
With expansion IM modules you can split into multiple rows of 8 modules. S7-312 only supports 1 row, max 8 modules. Higher CPUs supports up to 4 rows for a maximum of 32 modules.
As possible solutions, consider a 314 CPU, or to switch to IM151-8 + ET200S i/o. I recommend the latter.