DOS 7.0 (Released with Windows 95) and later versions are able to load the following CONFIG.SYS environment parameters in the Upper Memory Block (UMB) and free conventional memory.

The STACKS setting determines the number(0,8-64) and size(32-512) of the stack spaces that are created to handle interrupt handling in real mode (Note if set too low it can cause stack overflow in Windows) - Default Value 9,256.

STACKSHIGH=9,256

The LASTDRIVE setting specifies how many driveletters(A-Z) available to a DOS programn - Default your last drive letter.

LASTDRIVEHIGH=Z

The FILES setting sets the number of File Handles(8-255) available to a DOS program to have open - Default value 30.

FILESHIGH=40

The BUFFERS sets the number of disk buffers(1-99) available for limited buffering of file input/output (I/O) that is handled by MS-DOS real-mode drivers - Default value 30.

BUFFERSHIGH=40

The FCBS (File Control BlockS) setting specifies the number of FCBS(1-255) available to a DOS program, but most DOS programs uses filehandles - Default value 4,0.

FCBSHIGH=1,0

More Info MS KB82318
More Info MS KB145799
More Info MS KB274646