Michael Swan says in his grammar book:
<font color="#0000ff">I should think</font> and <font color="#0000ff">I should have thought</font> (also <font color="#0000ff">I would</font> / <font color="#0000ff">I'd </font>...) can introduce <font color="#ff0000">guesses</font>.
<font data-blogger-escaped-style="font-size: 12.0pt; mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;" style="font-size: 12pt;"><font color="#0000ff">I should think</font> we'll need at least twelve bottles of wine.</font><font data-blogger-escaped-style="font-size: 12.0pt; mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;" style="font-size: 12pt;">
<font color="#0000ff">I should</font> (<font color="#0000ff">I would</font> / <font color="#0000ff">I'd</font>) <font color="#0000ff">have thought</font> we could expect at least forty people.<data-blogger-escaped-o:p></data-blogger-escaped-o:p></font>
<font data-blogger-escaped-style="font-size: 12.0pt; mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;" style="font-size: 12pt;">This structure can also introduce criticisms.</font>
<font data-blogger-escaped-style="font-size: 12.0pt; mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;" style="">I should have thought he could have washed his hands, at least. </font>
<font data-blogger-escaped-style="font-size: 12.0pt; mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;" style="">(PEU 588)</font>
<font data-blogger-escaped-style="font-size: 12.0pt; mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;" style="">
</font>
<font data-blogger-escaped-style="font-size: 12.0pt; mso-bidi-font-family: Calibri; mso-bidi-theme-font: minor-latin;" style="">Are these about British English? Or true to American English as well?
</font>
The same goes for British English. This use of I should has an old-fashioned ring to it, even in Britain. We would probably also say I would or 'I'd'.
Practical English Usage was written in the late 1970s, so some of the language in it may sound a little outdated. It has been revised over the years ( most recently in 2016) but there are still a lot of early editions around.