Is every cell competent?
Assuming that you are asking about E.coli cells being made competent for transformation using plasmid DNA, an answer is cached here from this source:


  Hanahan, D. &amp; Bloom, F.R. (1996). Mechanisms of DNA transformation. In F.C. Neidhardt (ed.). Escherichia coli and Salmonella : Cellular and Molecular Biology. (pp. 2449-2459). Washington D.C., United States: ASM Press. 


The source (and it is perhaps worth emphasising that Hanahan is/was a transformation guru) states that treatment with divalent cations can achieve 10% of cells rendered competent, whereas treatment for electroporation can achieve transfer into 95% of cells.

This information was, of course, published in 1996 so it is conceivable that advances have been made in the case of the former method.