you normally have 1 copy of the lac promoter and 1 copy of the lac repressor (lacI) on the chromosome. The amount of repressor in enough to control the expression from the 1 copy you have on the chromosome, however, when you have a the lac promoter on a plasmid then suddenly there is much more promoter than the repressor can bind to. Think about it this way, assume you have 10 molecules of lacI which are sufficient to suppress 1 lac promoter. Now you have 20 lac promoters hence you require 200 molecules of lac to control it but the chromosomal copy only produces 10 and these will bind to the excess of promoter hence "titrated" when the lacI molecules bind 1 or 2 promoters the rest of the promoters that do not have lacI bound to them become active in absence of the inducer (IPTG or lactose). To overcome this many plasmid encode a copy of lacI, in this case when the lac promoter copy increase so does the copy number of the repressor (see pet16b for an example plasmid). Alternatively a single copy of chromosomal lacI can be sufficient to control expression from a multi copy vector if the promoter of lacI is mutated to produce higher than normal amounts of the protein (see lacIq1 promoter, with ref added below).

ref:

https://pubmed.ncbi.nlm.nih.gov/9858738/