I am getting the same warning from components/com_abook/models/book.php on line 58
This should be fairly simple to fix.
php 7.2 introduced a warning if you try to count($something) that is not countable. Typically an array is countable and the problems here are that in some circumstances the variable passed to count might be a single item rather than an array.
For 7.2 the simplest fix to to test the $something is an array before trying to count it and if it isn't setting the count to 1 (because it is probably a single item). There is a built in test isArray($something) which returns true if $something is an array and false if not.
Technically objects other than arrays can also be countable, so php7.3 has introduced a new function isCountable() which would be better to use if you are on php7.3 or higher as it would be more future proof.
I will try to remember to post a hack here when I have tested it, unless whoever is maintaining the Abook code gets around to it first. There may be other php7.2+ incompatibilities...
RogerCO