# Load required package
require(TxDb.Hsapiens.UCSC.hg38.knownGene)

# Assign package to variable for easier use 
txdb <- TxDb.Hsapiens.UCSC.hg38.knownGene

# Get promoter regions for genes (2000bp upstream, 0bp downstream)
df <- promoters(genes(txdb), upstream = 2000, downstream = 0)

# Get promoter regions for transcripts
promoters_txdb <- promoters(txdb)

# Get promoter regions for transcripts with 2000bp upstream
promoters_txdb <- promoters(txdb, upstream = 2000)

GRanges object with 6 ranges and 2 metadata columns:
                    seqnames      ranges strand |     tx_id           tx_name
                       <Rle>   <IRanges>  <Rle> | <integer>       <character>
  ENST00000456328.2     chr1  9869-12068      + |         1 ENST00000456328.2
  ENST00000450305.2     chr1 10010-12209      + |         2 ENST00000450305.2
  ENST00000473358.1     chr1 27554-29753      + |         3 ENST00000473358.1
  ENST00000469289.1     chr1 28267-30466      + |         4 ENST00000469289.1
  ENST00000607096.1     chr1 28366-30565      + |         5 ENST00000607096.1
  ENST00000606857.1     chr1 50473-52672      + |         6 ENST00000606857.1
  -------
  seqinfo: 640 sequences (1 circular) from hg38 genome
> 9869-12068
[1] -2199 # this is not 2000

If you use promoters and do not provide an argument for something that has a default, you will get the default argument. Your last call to promoters did not specify the downstream distance, which has a default of 200.

> suppressWarnings(table(width(promoters(TxDb.Hsapiens.UCSC.hg38.knownGene, downstream = 0))))

  2000 
278220 
> suppressWarnings(table(width(promoters(TxDb.Hsapiens.UCSC.hg38.knownGene))))

  2200 
278220 
>