The largest repeat count supported by GIF’s Netscape Application Block is 65535.
The loop field in the NETSCAPE2.0 Application Extension is an unsigned 16-bit integer, so its finite values span 1 through 65535. The upper limit follows directly from the field’s width: 16 binary bits can represent 65,536 combinations, from 0 to 65,535. Zero is assigned a special meaning—loop forever—leaving 65535 as the largest finite repeat value.
The count is stored in little-endian order, meaning the low byte comes first. The maximum therefore appears as the two bytes FF FF. This is distinct from the one-byte sub-block length fields elsewhere in GIF, whose maximum is only 255; confusing those fields produces the tempting but incorrect answer 255.
The looping mechanism was Netscape’s application-specific extension rather than part of the original core GIF89a structure. It became broadly interoperable through browser and graphics-software support. A decoder that does not understand the extension can still parse the surrounding GIF data, but it may ignore the requested looping behavior.