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WITNESS: item count (1 byte)
  signature length (1 byte)
  signature (71 or 72 bytes)¹
  pubkey length (1 byte)
  pubkey (33 bytes)

Note that using at least one segwit input adds 2 witness bytes to the transaction header, the witness marker and witness flag. Also, when there is at least one segwit input, the witness block must have a witness count item for every input, which must be provided as a 0x00 byte for non-segwit inputs. The overall transaction elements of a segwit transaction therefore are:

version (4 bytes)
witness marker (1 WU)
witness flag (1 WU)
input count (1 byte)
  inputs (variable size)
output count (1 byte)
  outputs (variable size)
witness item counts (1 WU × input count)
  witnesses (variable weight)
locktime (4 bytes)

WITNESS STACK:  
item count (1 byte)
  signature length (1 byte)
  signature (71 or 72 bytes)¹
  pubkey length (1 byte)
  pubkey (33 bytes)

Note that using at least one segwit input adds 2 witness bytes to the transaction header, the witness marker and witness flag. Also, when there is at least one segwit input, the witness block must have a witness stack for every input, which must consist of at least a witness item counter indicating that the stack is empty provided as a 0x00 byte for non-segwit inputs. The overall transaction elements of a segwit transaction therefore are:

version (4 bytes)
witness marker (1 WU)
witness flag (1 WU)
input count (1 byte)
  inputs (variable size)
output count (1 byte)
  outputs (variable size)
witness stacks (as many as input count)
  witness item count (1 WU)
  witness items (0–n of variable weight)
locktime (4 bytes)

A P2WPKH input requires a witness in the transaction's witness block:

A P2WPKH input requires a witness stack in the transaction's witness block:

¹ also see What is the maximum size of a DER encoded ECDSA signature?.

^{¹ also see What is the maximum size of a DER encoded ECDSA signature?.}