Maximum we can have 4096 columns per table, but exact limit depends on several factors, listed below:
utf8 characters require up to three bytes per character, so for a
CHAR(255) CHARACTER SET utf8 column, the server must allocate 255 = 765 bytes per value. Consequently, a table cannot contain more than 65,535 / 765 = 85 such columns.
.frm file that contains the table definition. The server uses the following expression to check some of the table information stored in the file against an upper limit of 64KB:
if (info_length+(ulong) create_fields.elements*FCOMP+288+ n_length+int_length+com_length > 65535L || int_count > 255)
.frm file that is checked against the expression cannot grow beyond the 64KB limit, so if the table definition reaches this size, no more columns can be added.
info_length is space needed for screens? This is related to MySQL's Unireg heritage.
create_fields.elements is the number of columns.
FCOMP is 17.
n_length is the total length of all column names, including one byte per name as a separator.
int_length is related to the list of values for
com_length is the total length of column and table comments.
SETcolumns, or use of column or table comments.
a) InnoDB permits no more than 1000 columns.