The market shares for four leading smartphone brands $($Brand A$,$ Brand B$,$ Brand C$,$ and Brand D$)$ in terms of the number of units sold annually in a particular region are reported to be $50\%,30\%,11\%,$ and $9\%,$ respectively. A mobile phone retailer in a major city within the region conducted a survey of $1,000$ customers to determine their brand preference and recorded the following sales data:

Brand A: $510$ units

Brand B: $300$ units

Brand C: $120$ units

Brand D: $70$ units

Calculate the expected cell frequency $($Ei$)$ for Brand A$.$