The main sense of +1 is the operation, 'add one', which can't have a plural. If you mean the numeral [positive one], it has a plural when used 'as a word'. Italics and the apostrophe would be used, so +1's, to distinguish it from + 1s meaning + one second.
– Edwin AshworthDec 23 '19 at 20:06
@EdwinAshworth Isn't it already plural? :) Tom plus-oned* Dick yesterday, so it's no surprise if Dick plus-ones Harry today. Tom and Dick plus-one folks all the time, but Harry isn't plus-oning them back no matter how much they beg him to.* it's completely regular! So the singular is he/she/it plus-ones, the plural is we/you/they plus-one, the past and past participle are both plus-oned, and the gerund/participle is plus-oning.
– tchristDec 23 '19 at 22:51
@tchrist Apparently, unnatural verbifications were not unknown in '45: '... And my dear we're still goodbye-ing'. Sounds like they're at the sales./ All the best.
– Edwin AshworthDec 24 '19 at 15:05