There are some limitations to the compiler :
You can perform only one calculation in a formula.
Good False
a = a * b1 a = a * b1 + c

Maximum allowed labels 5000
Maximum allowed variable names 1000
Maximum number of INTEGER/WORD variables 10*
Maximum number of BYTE variables 20*
Maximum number of BIT variables 120*
Maximum number of STRING variables Up to available external memory
Maximum number of ALIAS statements 128

*Depending on the used statements and the used variables of the other types.
A maximum of 32 bytes is used internally. This depends on the used statements.
The stack uses some space too. So it depends on the used statements how much variables you can use. In the worst case (32+16+8) = 56 bytes are used.

8 used bit vars will use 1 byte;
1 used byte will use 1 byte;
1 used integer/word will use 2 bytes;
1 used long will use 4 bytes;
1 used single will use 4 bytes;
1 string with a length of 10 bytes will use 11 bytes.

Maximum nesting :

FOR .. NEXT 50
IF .. THEN 50
DO .. LOOP 50
WHILE .. WEND 50
SELECT .. CASE 25