One major difference between the variables of a programming language and registers is the limited number of registers, typically 32 on current computers, like MIPS.
This is what I read while studying MIPS language. Assume that there is a program which has more than 32 variables and each variable stored in a register. What will happen the rest of variables while there is no empty register?
Not all MIPS 32 registers are used to store variables, some has special uses like $k0, $k1, $gp, $sp, so the available registers for storing variables are less.
Usually variables are not stored in registers, they are stored in memory by creating a stack frame for the function like here:
# make space of 30 vars, 4 bytes each
addi $sp, $sp, -4 * 30
# $t0 = 1
addi $t0, $0, 1
# store vars into the stack
sw $t0, 0($sp) # 1
addi $t0, $t0, 1 # $t0++
sw $t0, 4($sp) # 2
addi $t0, $t0, 1 # $t0++
sw $t0, 8($sp) # 3
addi $t0, $t0, 1 # $t0++
sw $t0, 12($sp) # 4
#...
# now to access the 4th var, 12($sp), into $t1
lw $t1, 12($sp)
# do stuff
# deallocate the space and destroy those vars when you done
addi $sp, $sp, 4 * 30
when you want to update the variable or use it heavily like in counters, you load it from the memory into a register, do what you want to do and then update the memory stored variable.